ROB only got a few major changes-
Fall speed increase, factoring in the FF speed increase with it to, this is like 1/3 buff 2/3 nerf, but it's definitely warranted considering ROB was like a top 5 character for escaping punishments. Since his gravity wasn't increased, it doesn't affect SH/FH aerials much at all unless you FF during them.
Dthrow angle was lowered from 85->80. This like his fall speed change is kinda a buff but mostly a nerf. His chaingrabs on mid fallers/semi-fast fallers haven't changed, in fact if they DI away the CG gets longer than it did before. However, they can no longer DI behind him very far at all, which means ROB has more control over what direction the follow-up sends him. But the dthrow change makes following up on floaties very far as he has to both run far and jump/boost high to follow-up with an aerial (before he didn't really have to run for more than one frame if at all) and on spacies, his chain grab windows all got smaller and much tighter, and his follow-up opportunities are much worse as well. Thank goodness nobody is allowed to have a good matchup against fox! /PMDTDesignPhilosophy
Nair kills like 10-15% later out of a boost, and it also has a weak hit frames 23-28. The weak hit isn't bad, but the strong hit (which used to last the whole duration of 16-28) broke CC past 0%. SH nair as a spacing tool used to be safe if done in place or retreating as the hitbox you would usually land on a shield was like -5 or so given the right auto-cancel timing, you just had to be outta grab range and you were golden, it also did relevant shield damage and beat CC, and moved ROB out of the way for things like dash attack/grab approaches. With the nerf, the weak hit does 12 damage now, making it less safe on shield, and more significantly, doesn't break CC until much later.
Side-b used to be effectively his dash attack, and it also reflected, which was nice. His actual dash attack is pretty bad as an approach even for a dash attack, but side-b if spaced well could beat CCs and cover tech options, and was his main grounded burst range. In 3.6, it's just a reflector and moves almost nowhere; it can't be used as an approach, it doesn't cover tech options, and it went from multi-hit with a reasonable finishing hit to a weak single hit with an even weaker lingering hit. It's pretty useless except against really projectile-happy opponents, but even then the reflection box is spotty (idk why i get hit out of it sometimes, this was an issue in past versions too) and has more endlag than most reflectors do with no way to shorten it.