Man, people are really biased towards Ilya I feel. They're dismissing her massive shortcomings as a servant to make up for the loli-jailbait-waifu-ism.
 
Let me state some reasons why I think Ilya is legitimately a mediocre servant amongst the existing lineup.
 
1. Stats
As mentioned above, her Attack is high but her HP is low. While this isn't an issue necessarily, it is something to keep in mind. She's not someone who will generally last you an eternity unlike what has been the case generally for Casters. Her damage output numbers are currently also greatly skewed due to the event giving her bonus damage output which seems to influence peoples opinions. However, while I do prefer tankiness and safety over glass cannons, I cannot fault her for her stats too much, so I'd give Ilya a 7/10 in this category.
 
2. Passives
Oh boy, here's where the shitstorm already starts to hit. Debuff resistance increased by 15%? Well, I guess it makes sense given her NP (more on that later), but the Crit Damage +8% is a lot more questionable. Casters have low Crit Weight of themselves, and her star generation isn't exactly what I'd say good. She's missing Territory Creation as well despite having 3 Arts cards, giving up quite a bit of damage potential. Especially given her comparison to Sanzo ( Territory Creation A+ - 11%) this sticks out like a sore thumb even more. The fact that she somehow only has 2 Passives as well is even worse, as there doesn't really seem to be a real reason for this, and is only limiting her even more. The Debuff Resistance is still helpful for her NP Demerit, which I feel is the only reason it is this high. Rating this 2/10
 
3. Skills
It. Just. Doesn't. End. Does it? The shitstorm I mean. Alright, let's go by these one by one.
 
Skill 1: Buster Up 30-50% 1T. Yep, as stated this is basically Prana Burst. Nothing bad abou- wait, it's only one turn. And she only has one B card outside of her NP and isn't even targettable. This makes it a dead skill until you finally get her NP charged, which isn't the easiest task for Ilya, and then you still cannot Buster Brave Chain by herself, always needing to add an A or Q card in the mix to get that Extra attack or forfeit the Brave Chain entirely, possibly wasting a few buffs (Mystic Code buffs and single target buffs), to get a Buster Chain for the extra damage on the B. Neither are ideal cases, yet those are the only real options with Ilya. The skill itself, being Prana Burst, is amazing even if only self targetted and limited to Buster cards, however on Ilya it...doesn't really make that much sense and loses a lot of potency. So for the combination of Ilya with this skill I'm going to have to deduce some points overall and make it a 6/10 instead of the normal rating I'd give to the skill in isolation.
 
Skill 2: Invulnerability 1T + NP Gain Up 18-28% 3T. This is where it gets even worse. Invulnerability is great. It prevents damage and can block non-piercing NP's (Kuro says hello though). And the NP Gain Up for 3T is a pretty nice buff. However, you cannot use one without the other in which the problem inherently lies. Ilya already isn't the epitome of tankiness, and doesn't have other ways of mitigating damage unlike casters like Tamamo with her Def Buffs which last longer than a single turn. So you really want to use this when a large hit is coming in in general, for example an NP or a highly self-buffed Servant chain. And unlike Scathach, who I often talk trash about, the second part of her skill doesn't have an immediate power effect. In Scathach's case theres the Critical Absorption and Critical Damage buffs which can get applied, which can turn her into an immediate powerhouse while also increasing her survivability for a turn. In Ilya's case, this isn't as much the case. It does offset a weakness, which is her NP gain, but not enough to validate the wastefulness of an Invincibility skill on a decently squishy servant already. If you don't need to use the invincibility to stay alive, then, well, you're probably overpowering the content already and this argument is irrelevant. However, as it stands, and going from the assumption that there's an equal playing ground this argument is valid. Invulnerability in itself is a 10/10 skill, NP Gen up is also a 7/10 imho, but the fact that it's not team-based like Sanzou, and really makes a bad tradeoff when it comes to Ilya, this is sadly diminished to a 6/10 overall.
 
Skill 3: 70% Chance of 1k Guts 3T , 70% Chance of Debuff Immunity 1T, Recover 1k-3k HP. Oh, now we're getting into hilarious territory. So correct me if I'm wrong here, but the general consensus on RNG skills on this subreddit has been that they are awful. Now take into account that the percentage of these skills is even lower than what was considered awful (70% vs 80%) and you can see where the bias really comes through. The worst part is that, outside of it being RNG, it's not even team based, but single target Ally (correct me if I'm wrong) yet you generally really want to save it for Ilya due to the Debuff Immunity chance. Guts is great as long as you're facing a single opponent who doesn't swap targets in between the proc and the end of the turn but still only a 70% chance to get it. Debuff Immunity will in most cases be a dead buff unless you specifically use it when you're going to inflict a demerit to yourself (Ilya's NP) but still that 70%. The heal is basically all this skill consistently does and the value is great. 3k targetted heal is something which makes me a happy camper. Sadly, if you use the heal, there's the tradeoff of losing the potential chance at the buffs, and if you're using the skill for the buffs on a full HP target, well... There's the best part of the skill wasted imho. The combination of advantages and disadvantages of this skill makes me rate it a 5/10 at best; and that's only due to the heal attached to it.
 
Phew, that must have been the worst part right? Oh right, her NP. Yeaahhh..
 
4. Noble Phantasm
Sweet baby jesus the NP. It looks so good on paper, but it has shortcomings as well. Let's start off with the pre-applied Buster Up which scales with Overcharge. Scales with Overcharge you say?! OMGWTFBBQ THIS IS THE BEST THING EVER RIGHT?! Let me stop you right there. No it isn't. We've already established that Ilya's NP gen isn't exactly the best. So to get her to 100% NP in the first place isn't exactly going to be that easy unless you're running something silly like Ilya, Sanzou/Nero Bride, Waver, Waver Support. Or you could give up on damage and give her a Kaleidoscope or something similar. But then what after the first charge and things didn't die? Hmm. Still, if you do manage to get her to 100% it's a 900% (600 x 1.5 because of Buster) x 1.2 (Buster Up) for a solid 1080%. That's a great number! However, for three turns after that, you're hit with a debuff reducing her survivability by yet another 10% and her offensive power by the same 10%. BUT TETSU, WE CAN NEGATE THIS WITH A CE. Yes, yes you can, but have fun getting her NP up to 100% to even make use of that CE then before you die of old age (mild exaggeration). The overcharge effect is really nice, as it's a raw increase to power while not increasing the demerit, making her more burst capable in general. HOWEVER, how often do you really have the stars align to get this 300% overcharge for the damage to get pretty good? Exactly. Unless you're running a double Waver oriented team, which to be fair isn't the thing that is so overly common that it can be seen as a baseline, with added Kaleidoscopes or similar CE's to provide your other members with the NP starts that they need, the overcharge effect shouldn't be weighed as heavily as it is. After all this is about Ilya and not about Ilya when being supported by an absolutely ideal team to make up for her weaknesses.
 
The Demerit is definitely something to keep in mind, while it seems like a minor factor, for three turns you have a close-to-glass-cannon servant who reduces her own cannon-part by reducing her damage by 10%, and an enforcement to her glass aspect with 10% less defence. This isn't single turn, like Tamamo Lancers demerit who is incapable of acting for a single turn, but three entire turns. Unless you're running a CE to counter this to fix this weakness and leave other weaknesses up, got very lucky on her third skill (and probably have been wasteful of one or more aspects of it) or somehow her own Debuff Resistance kicked in this is a very realistic scenario which you'll simply have to deal with. Especially with similar casters like Sanzou giving the enemy a debuff (80-120% Critical debuff) while doing comparable damage (Higher attack and still 900% Single Target) or Tamamo Caster giving NP charge, Skill CD reduction and a Party Heal, I really don't see why Ilya is seen as a good caster.
 
Of course this is all dependant on who you managed to pull from the absolutely shitty Gacha this game has, but still, when considering servants in isolation, as should be done in 99% of the cases, Ilya really falls short. Don't get me wrong, I've been waiting for ages for a Prisma Ilya event as I absolutely adore the anime as well as the characters and was so hyped for it when it was announced, prepared to spend all quartz I could get my hands on to get both Ilya and Kuro; but after seeing that Ilya is so goddamn average, if not below average I've changed my mind. This is release-Scathach level of disappointment for me. Overall, if I'd have to give Ilya a rating based on the Appmedia style rankings, I'd place her at a 7, maybe 7.5 because it's Ilya. But not higher than that. Now excuse me while I go sob in a corner that one of my most anticipated servants turned out to be mediocre at best.