I use both Unreal and Unity and they both have huge issues imho, but the only thing you can do is live with them (or try to fix them with plugins or workarounds), I could make you an endless list of user experience problems in both engines (Probably I know more in Unreal because I use it more).
Epic is focusing too much on new features and sometimes it’s breaking old things that were working (Imagine that physics simulation was broken in 5.0, the widget favorites menu is more broken than it was before I opened a bug report, despite they marked it as “resolved”), you should definitely lower your expectations if you think it’s flawless 
Oh, try packaging a project for Android (even better if it’s VR), you’ll learn what frustration is 
(In Unity is so much easier)