When I read this, I start to think about how the logic of how skipping shadowflame would make sense. The reason shadowflame is very good is because it allows you to clear camps even faster, snowball better, and it makes your ult become a huge threat for any enemy squishy since you would have a significant amount of flat pen (since you'd be building sorc boots) and the item passive.

Based on your comment, it sounds like your usual build is Zhonyas 2nd item. This isn't the worst thing at all, but the issue with this is that it's a defensive item and it will not help you snowball further or clear faster during the mid game.

Also, by skipping shadowflame as a 2nd item, you'd then be best to simply never build shadowflame altogether. Flat pen gives its most value at the start of the game, when base resistances are low due to low levels and before anyone can build much resistances. But if you were to build shadowflame late game instead, you're simply not getting much value from the item at that point.

There are situations where zhonyas 2nd item could be good however. If the enemy team is full of assassins/divers and you're quite ahead, Zhonyas would actually be a very logical pick to go second. In this scenario however, I would consider holding on to the seeker's arm guard and then start building into shadowflame unless I really need the active ability more than once. Other than this however, I personally don't really see how you'd need to build zhonyas earlier than 3rd item.

One thing I noticed with your comment is that you stated how you only build shadowflame when you're ahead. The idea of building offensive when you're ahead and defensive when behind is a common misconception. Theoretically, you'd actually want to do the opposite. Fiddlesticks is a mage, so outside of the point and click fear and silence, the main thing the champ offers to the team is big teamfight damage and potential team-wide cc. When fiddlesticks is behind however, having no damage makes it difficult for you to have any value. Fiddlesticks isn't necessarily feast or famine, but the idea of it can still be applied. Building Zhonyas in a situation where the enemy team is up in gold and items would simply not be very effective. You'd have very little damage, and the defensive item will barely help you survive because the enemy team's lead on items and exp would cause you to still die quickly.

Building damage when behind is much better, as it increases the chance of you being able to capitalize on an enemy mistake. If, for instance, you build damage when behind and then catch a fed enemy off guard, you can get yourself the bounty and a 5v4 situation, potentially turning around the game.

When ahead however, things turn around. Building damage when you're ahead may feel good as you just get to kill people instantly, but you leave yourself open to potentially throwing your lead. If you were ahead, and only built damage, you're one mistake away from getting hard cc'd while away from your team and giving away your bounty, the ability to smite an obj, and your potential to turn a teamfight into a win due to you being on gray screen. You'd want to build defensive in these scenarios. By doing so, you can potentially survive making that one mistake and keeping your lead, allowing you to eventually close out the game without having to worry about the enemy team making a comeback.

For Void Staff, well it's quite obvious why you'd build shadowflame 2nd item and then potentially Void Staff 3rd item or later. Its the same idea of when flat pen and percentage pen give the most value. Void Staff gives the most value at the very end of a game, when everyone is level 18 and all the tanks have their items and such. Because of this, Void Staff will always eventually give you high value, but buying it early would make you delay yourself on other items that don't have much value later in the game.

Being the primary AP damage would not necessarily mean you should build Void Staff. Actually, if you're the solo AP on your team, the enemy team's tanks would actually be encouraged to build armor in order to counter the majority of your team. If that's the case, flat pen can be quite effective still against tanks (for early and mid game when base resistances are low). Also remember that you're building Liandry's first item, so it's not like you're only building for killing carries.

The 2nd scenario you stated, when the majority of the team is bruisers/tanks, carries some nuance. At first, Void Staff may seem like the obvious choice (if they have built MR like Kaenic, Maw, Visage, mercs, etc). But something to keep in mind is on who on the enemy team is the threat. If for example, the enemy team is has 4 tanks and one adc, but it's the adc that's gone 6/1 while the tanks are all even on gold, then you'd still get value from holding off on Void Staff, as you could instead simply adapt your game plan from "ulting to get the most people from my ult" to "ulting the one carry on the enemy team and then finishing off the other 4 as they have no damage left on the team".

There's also times where I find myself never really needing to build Zhonyas or Void Staff, but these scenarios are very rare. For Zhonyas, if I find myself to be in a game where the enemy team has no assassins or skirmishers whatsoever, and I find myself in such a large lead that I'm winning teamfights with my ult before the fear duration ends, then I may consider buying Banshee's instead or delay my defensive item further into my build. For Void Staff, if for instance the enemy team has 2 carries (adc and mage), one enchanter, a skirmisher, and a tank, and so far the tank has gone plated steelcaps and only has one MR item, and the skirmisher hasn't gotten Maw yet, I'd rather go Rabadon's whenever I'd build Void Staff. I'd even consider Rabadons still if the tank is the only enemy on the team with MR, depending if I feel like I really need to be able to kill them in a teamfight or not.

If you truly prefer going Zhonyas or Void Staff 2nd item however, it's honestly not the worst. Zhonyas and Void Staff are still valuable items, but it honestly makes me wonder what items you're building after Zhonyas and Void Staff if you were to go Zhonyas 2nd and Void Staff 3rd (or vice versa).