CalDAV works great! I use their calendar on my Windows Mobile device without a problem[0]. FastMail even, when they had a change that might negatively affect some calendar clients last month, bothered to test large number and notify users who might be adversely affected. I got an email telling me how to resolve the issue with my Windows Mobile device, for example.

https://blog.fastmail.com/2017/11/04/shared-calendar-improve...

[0] Actually Windows 10 has a strange quirk of CalDAV/CardDAV support: It's only supported as part of iCloud account support. The solution is to create a fake iCloud account, and then change the advanced server settings to your CalDAV and CardDAV URLs instead of Apple's URLs. And then put in the correct credentials. This is true of desktop and phone Windows 10 alike.

I haven't had that experience — I have it hooked up as CalDAV to my mac and iphone's native clients.

Hmm, maybe I'm just doing it wrong.

I found the Fastmail calendar to be far more reliable and compatible than Gmail.

Check out DAVdroid if you are on Android (there are other options too). Provides CalDAV/CardDAV to sync with Androids native calendars and contacts.

Maybe try again; I use the calendar with Mac and GNOME native calendar programs just fine