Did you use the 'litmus' test suite? I found it very useful when building Fastmail's (perl) WebDAV file server implementation.

There were also a bunch of fun things with quirks around unicode filename handling which made me sad (that was just a matter of testing against a ton of clients).

As for CalDAV and CardDAV - as others have said, JMAP Calendars/Contacts will make building clients a lot easier eventually... but yeah. My implementation of syncing as a client now is to look for sync-collection and fall back to collecting etags to know which URLs to fetch. Either way, sync-collection ALSO gives a set of URLs and then I multi-get those in batches; meaning both the primary and fallback codepath revert to the multi-get (or even individual GETs).