I have developed for iOS for the last 5 years, and I haven't ever experienced those in UIKit. This year brought one single bug in UISegmentedControl, in a situation that doesn't happen often: https://forums.developer.apple.com/thread/122736
UITabBarController—https://twitter.com/LeoNatan/status/1168286996915204097
UIActivityViewController
Various UIView layout issues
Safe areas—https://twitter.com/LeoNatan/status/1179475940784721920
UITableViewController—iOS 13's sheet presentation broke clearsSelectionOnViewWillAppear (https://twitter.com/LeoNatan/status/1137474148488364034), for example, but it's often broken: https://twitter.com/Javi/status/644631570083483648
SLComposeServiceViewController is completely broken on iOS 13
Perhaps you are exclusively referring to mobile 'apps' and the APIs you can use for those?
If you are using features of the frameworks that are outside of the "happy path" for that year's keynote announcements, you can expect terrible bugs on a yearly basis. These bugs happen for the exact same reasons as stated in the GP.