The reason iOS 11 sucks in my opinion, is that Apple has a finite number of iOS developers, and this year, they needed to invest tons of work in iPhone X UI + Gesture work + Hardware support + Face ID + New GPU (while in the previous years, things were kinda stable / iterative on most fronts).
Since they work on one iOS for all devices, they had much less attention for bug fixes and stabilization, and they have a 1 year limit because of the new device cycle.
This update model sucks, because when there are big hardware changes, the software is less mature even for older devices, and they don't back port security issues, so you have no choice but to update...