My biggest issue whenever I'm using Android is how inconsistent it all feels. Sure, vanilla Android looks good and one could argue that it doesn't really look any inferior to iOS at all at this point. Rumours has it that when using Google Pixel phones the UI acceleration is on-par, if not better than iOS so its all smooth and dandy.
But whenever I use Android its never vanilla Android. It's most commonly Samsung with their TouchWiz / Grace / Samsung Experience UI and even though it has improved over the years its just not all that great compared to vanilla Android and iOS and even with theme's you get this inconsistent theme's as third-party themes has various elements they can't change making it all looks horrible if you ask me.
All those handy, dandy features? No guarantee you will ever see them on phone's not running vanilla Android. Updates? Forget about it. It's a mess... Android would have been in a much better state if Google would just enforce vanilla Android on all handsets. Sure, let the manufactures keep offering their own bloated, never getting updates solutions but require the option to always have vanilla Android as a option.
Third-party apps is the same inconsistent mess. There is no enforcement's whats so ever so the UX-design, the navigation and behaviour in third-party apps is all over the place. There is no enforcement's requiring developers to adopt new API's from newer versions of Androids so you will mostly see all these new and fancy capabilities added to Android, but no third-party apps starting to make use of them for years...
Apple has also become more sloppy with their enforcement's, but its still leaps beyond what Google is doing with Android and it makes the whole experience a mess. I have yet to see any third-party app work better on Android compared to iOS unless it something that is simply not allowed on the Apple AppStore to begin with. Even huge apps like Snapchat keeps taking far worse pictures and navigate slower on top-of-the-line, brand new Android devices. Same goes for popular apps like Facebook Messenger. How is this even a thing in 2018?