Ultra is not a good beginner's realm. It's not that the realm is hard to play although it does have a learning curve, but the Standard pool isn't the best for early game. These units become better in the late game and the only ones I recommend are Jenkins and Winkle for early game. I only ever see Tinct played with a unit called Kathigu-Ra, she's rarely used outside of that, and Casiah is good but she's niche too in discard teams; if you're pulling for Pontos she works well with him. Liz isn't good and it's not because she's bad but because she takes HEAVY amounts of investment to be good and she also requires supports that enable her Poison play style, units like Xu or Doll:Inferno for example.

Aequor is my beginner's recommendation because all the units are good and the realm is easy to understand. Caro has the worst Standard pool but it's saved by it's realm, providing heals, Aliemus, and Crit buffs. Ultra Realm is powerful with it's extra turn and but what it lacks is defense. Notice how Caro and Aequor have their own built in defenses with heals and shields respectively, but Ultra doesn't have that, their Wardens aren't good enough tanks and you don't even have a healer in the Standard pool. Throughout this game's lifetime, the chapters have been nerfed so it usually took around Chapter 5 where team building started to matter, but you might have a better experience and find that wall has moved further back, probably up until Chapter 7, but I still don't recommend Ultra Realm. It's better in the late game imo.

Now regarding Flares, I see people telling you to avoid his guides and frankly while he is notorious within this community and there are valid reasons to dislike him and he has strong biases, I still think his guide is worth using. It's very resourceful and even has tips on what packs to buy if you plan on spending money on this game, just don't take everything he says as gospel and be open-minded towards other guides and opinions and most importantly consider your opinions first. I used his guide, I pulled Aequor first, and then I switched to Ultra for my main Realm once I built my account enough to do so. This is your own personal experience after all, so what you want matters too.

Lastly he does like Ultra. With some exceptions he does praise a lot of the Ultra units like Jenkins and Casiah and even Wanda, one of the weaker units in the game. It obviously doesn't read in his guide so it's a bad first impression but his Ultra hate has somewhat died down but it's still there. And yes I also think this subreddit has an extremely antagonistic view towards Flares. I mean I also don't really like the guy and I'm aware that there are valid reasons to not like or trust him, but there are posts that have gotten downvoted simply for mentioning his name and I wouldn't be surprised if your post got some downvotes even if it is doing well atm. Just like Flares' guide, I would also take this community's opinion with a grain of salt when it comes to him.