It's on more fronts than UI design (which is still declining). Overall performance is bad, and updates seem to make things worse every time. My parents are both on older iPads (my dad has an iPad 2 and my mom has an original iPad Mini) and they have both refused to take any further iOS updates past 7. It's a good thing they're sticking to the old software too, because my wife's own original Mini is almost unusable now on iOS 9. We're talking 5-10 seconds of lag from when the keyboard comes up until it starts working. Every time an update comes in, I hope that they've fixed it, and every time, it only makes it worse. But there's no way to roll back so I'm stuck.
Apple has leveled a lot of criticism at Android phone makers for not supporting their phones with OS updates, but I'd rather be stuck on an old version than forced (with constant, annoying nagging) to update to a new OS that basically ruins my device.
Combined with the new "Error 53" BS, Apple has successfully gotten my parents, who have been loyal to the company since getting their first Mac in 1986, to completely swear off iOS devices. My wife and I too. My parents are using cheap Moto G Android phones, which cost about $50 each without a contract, and they are very happy. They actually DO "just work."
I switched away from Macs a while ago, but my parents are still sticking with them... for now. I think they're running Snow Leopard (or is it Mountain Lion? One of the two) and they're too afraid to upgrade because they haven't seen a good new piece of Apple software in a long time, and they actually use their Macs for work. I certainly can't see any reason for them to "upgrade" beyond where they're at now. My dad has already told me as soon as he retires, he wants to just get rid of all his computers and gadgets and lead a simpler life. Can't say I blame him.