I did play Generations on PS3, and it was awful, but that was mainly due to the bad control (Which I assume has to do with the framerate but didn't the PC version come until later? Either way it was my only option) and sonic constantly blasting off of ramps in 3D mode while 2D Sonic felt way too stiff. I also played Generations on 3DS before then and didn't like it because of how short and easy it was, but looking back it actually controlled a lot better as the 3D sections weren't really much different from the 2D ones (although that also makes it not have as much variety) and Classic Sonic had a homing attack which made his easy stages even easier.
My main issue with FPS is when a game is made for a higher FPS in mind, only for it to be downgraded and suffer because of it. If a game is made with 30 in mind with nothing else, then I'm fine with it, same for if the 30 FPS experience is barely different than the 60. I watch 30FPS gameplay videos all the time and the differences in viewing are minor, but when you play a lot of said games then it makes a difference. Heck, OOT, one of my all time favorite games only runs in 15 FPS on the N64/30 on the 3DS but I enjoy both versions as they were made with the system in mind. Sonic Forces at least tried to get the 60FPS control on the switch but when I played the demo on the Switch and then the PS4 one, it was obvious which one was smoother feeling. If a better version of the same game exists, then I'll buy it if it's on a system I own, which is just what I did with Forces and found it to be enjoyable for the $40 (any more though and I wouldn't really care for it as much due to the length)
Most of the time I don't even mind slowdown in retro games, as long as it isn't to the point where the whole game constantly moves at a super slow pace like Turbografx 16 Gradius, or makes the controls screw up and cause me to die.