For me, a big one is integration with email / calendar / contacts services that aren't Google. I don't know where Google dropped the ball here - Android was originally amazing for this kind of thing - but at some point they started bolting a lot of features specifically on top of Google accounts, and out of the box Android doesn't even understand how to sync with CalDAV / CardDAV. So if I want my Nextcloud stuff to work at all I need to go and install a third party app. The third party app works great (I happily used DAVx5 for many years), but it's ridiculous when iOS has all that integration officially supported and available straight out of the box. And it even does clever things, like suggesting contact details it learns from my (Fastmail) email. Android has that stuff, but it is completely on the cloud, and it only works if you give everything to Google.
I sort of get it. Outside of Gmail and Exchange, mail with calendar and contacts is a bit hit and miss. There just isn't the all ecompassing protocols like Exchange that can cater for it all, so you have to use the "niche calendar protocol from 2007" to fill the gaps.
I pay for O365 mainly for this purpose, as Exchange is the defacto mail provider of todays age. I used to host my own Exchange but in the last couple of years and vulnerabilities kept on getting more and more worrying and the patches became more involved, so I just decided to pay MS for the service. Perhaps I played right into their hands ...
True words. Like anything else though, if you want niche - you get niche. You've got to put in the work yourself. I assume apple supports calDAV better because they stole the protocol and based their own calendar events system on it.