You can expect the iOS implementation to be buggy and not fixable without an OS update. Meanwhile, there are multiple open source CalDav implementations for Android that update automatically.
The most probable reason it isn't installed by default is that CalDav is a failed standard, which most people don't use and don't want wasting space on their devices. ActiveSync and Google Calendar won, which is why even iOS paid the Microsoft tax to support ActiveSync long before it supported CalDav.
I used CalDAV on both iOS and MacOS for the last 2 years and worked just fine. Never had any problems. Feel free to correct me if I'm wrong.
It's true that back in 2015 FastMail was not doing invitations and RSVPs through CalDAV, but they fixed that since then.
> most probable reason it isn't installed by default is that CalDav is a failed standard
That is extreme bullshit, the same kind of bullshit that Google used to kill Google Talk and its XMPP interop, while pushing for their shitty and completely proprietary Hangouts.
People used Google Talk back in the day due to its simplicity and interoperability. Who is using Hangouts now?
I actually hope it dies, because we can then put such bullshit arguments to rest.
CalDAV can only become a failed standard because unpaid fanboys and shills keep normalizing this behavior.
But until then I'll keep using CalDAV and I'll be happy about not participating in this charade ;-)
(when I say "we" - I haven't actually attended a CalConnect for a year, due to spending half my life traveling as it is - but FastMail has sent someone to every event for the past few years, and Ken has been going for ages before that)
The most probable reason it isn't installed by default is that CalDav is a failed standard, which most people don't use and don't want wasting space on their devices. ActiveSync and Google Calendar won, which is why even iOS paid the Microsoft tax to support ActiveSync long before it supported CalDav.