I wanna sell my iPhone X and get a flagship Android phone (such as Oneplus 8 Pro), what will I miss?
Last Android phone I owned was a Note 3 and it was pretty shit.
Galaxy Notes have been good starting with the Note 8, the Note 7 battery explosion debacle really packed Samsung's shit in. They also don't use cheap RAM or whatever made their older Androids crap out in like a year, so there's that. Then again, Galaxy Notes also cost as much as high-end iPhones too so if you don't wanna spend serious skrilla, don't bother.
Everything I used on iOS had an Android equivalent, there doesn't seem to be much left that's exclusive to one platform or another anymore. The smattering of things that are Android exclusive are always because of Apple's autistic control over their app store, like how they won't allow remote game streaming services or emulators.
You might miss Apple's keyboard. Maybe it's just me, but for whatever reason, iOS has always been unparalleled for phone keyboards. Samsung's keyboard is alright, and SwiftKey's the best one I've found.
Camera quality for Androids tend to be a step or two behind iPhones, though I'm not sure how much that matters at this point because even flagship Androids from two years ago can shoot reasonably competent 4k60 footage, though then again phone cameras never adopted optical zooming, but instead just slapping on multiple fixed lenses at different zoom levels and the quality can be all over the place. Check reviews and comparisons first if that's important to you
And if you're clumsy, it's a good idea to look into availability of replacement parts and just how repairable the phone is. Apple stuff's extremely popular, so it's trivial to find replacement screens and batteries, but Androids can be all over the place. Samsung stuff is readily available, I'd imagine OnePlus probably is too, considering their popularity, but that's a serious danger if you go with an obscure brand.