Remake was designed so that poor play could be mitigated by burning resources, while hard mode locked out most resource usage forcing you to master the game. I felt this was a fun balance and enjoyed both playthroughs. Rebirth however is balanced so that mastery is essential for normal play. This isn't entirely a bad thing, but the rewards of playing well are a lot less this time considering this is just the boring middle portion between anything interesting happening, while the spectacle of seeing old blocky PSX graphics recreated in modern style is no longer impressive.
There were a bunch of specific bosses that stood out to me that would make me rethink any second playthrough:
-Vincent: He's aggressive in all the ways that play badly, but he is punishable with normal strategies. The issue is you're forced to use Cait Sith for this fight who may as well have been designed for an entirely different game. I know a lot of people were wondering if and how they would implement Cait Sith, and the way they chose works about how you would expect. But the enemies in Rebirth are all from a DMC game and he's just not designed for that. The entire Shinra manor sequence made me loathe the character and this was just the kidney stone on top of a turd sandwich.
-Odin: He's optional, to be fair. The way the fight works, he kills your whole team if he ever hits you once with any of his attacks. That is a mighty slim margin of error in a game with this much attack spam. And to make it worse, you can't even perfect guard his attacks. A block still counts as a hit and you get Zantetsuken'd. So you can either learn every one of his (large) moveset and then perfect dodge everything, or just... aggro switch. As long as he doesn't hit the character you're controlling, you're fine. So use Barret/Aerith/Yuffie and plink at him from range, switching every time he turns on your current character. It feels so dumb to play this way, but the alternative is bananas.
-Roche's final fight: The first point where the game really says "You must master this one mechanic or you fail." Roche counters everything unless you perfect guard him, so I hope you've been practicing. I had the perfect guard materia maxed and had been practicing perfect blocks the whole game, but I still had a hard time consistently guarding his attacks.
Roche is a good example of the death spiral problem. If you mess up a block you're going to eat a lot of damage and need to heal. But to heal you need to deal damage, and if you had the ability to deal damage to him you wouldn't need to heal. There's no shortcut here, you have to learn his timings. There are a couple materia that can help (Item economizer and the one that gives you ATB for blocking). It also bothered me that even when you managed to score a hit on him he would shrug off the combo after 2 attacks, meaning you can't even get one full combo on him. You have to get multiple perfect blocks in a row to even get to the point where you can punish him (I guess that's what a True SOLDIER is like) This fight would really be improved if Cloud had some kind of charge or Taunt move that let him get ATB without attacking.
-Rufus: The great filter. So many people quit on this boss. You just mastered perfect block, and he ignores it. None of his attacks can be countered. Instead, you have to hit him when he reloads. It only takes him a second, so if you try to attack on reaction to reloading, it's already too late. The trick is he reloads after every gun-based special move, so you attack just before he finishes the move. I actually found this fight pretty fun, but it's another DMC fight--in fact I would say most DMC bosses have more flexible strategies than this guy. It really is just doing that one thing over and over again. However, the big Fuck You was his stagger resistance. I actually killed him before he staggered, despite using Focused stab exclusively whenever I could. So even when I had it down pat the fight just dragged on forever.
-Final boss: As I said, it's a many staged affair and it has mid battle checkpoints. I view this as a tacit admission your combat system is broken and you can't be bothered to fix it. May as well just give the player unlimited HP at that point. If the final fight is an elaborate cutscene of kingdom hearts bullshit, there's no reason to try to pretend player skill matters. I kept thinking of FF3 on the NES, which places its final boss at the bottom of two final dungeons in a row with no save points, filled with one brutal encounter after another. Even when you excuse "nintendo difficulty," that's what it means to commit to your game design.