I am a newer iPhone user (2 years now) and I am of the same opinion as you. I see so many people crying foul, that their phone is now unusable, but I’m just hear going “eh it’s uglier” and continuing. Curious what OP thinks is fundamentally broken.
Now I have heard about issues on MacOS and things but not really anything around the phone.
Something I've heard from someone that owns an iPhone 16 Pro is that animations are (were?) laggy sometimes. I was also looking at some pictures on an iPhone the other day (unsure about the model, maybe 14?) and it felt like it was dropping some frames while switching apps.
So while it may not be fundamentally broken, it's the type of stuff that would annoy me a lot if I used an iPhone. I never expect to go from smooth experience to a low-end Android phone experience after a software update.
MacOS... I've avoided upgrading my M4 Max MBP so far after upgrading the M1 Air we have at home. It's just not as smooth as before, even with reduced transparency.
iPhone 16 Pro and Apple TV and Apple Watch have all become noticeably slower/laggier with choppy animation under 26. Totally unacceptable.