I think the biggest issue is that if you are putting that effort into syncing, its far nicer for the end user if you manage it for them.

If your using the "local" shared storage, then you're on the hook for any failure or setup, so there is a huge amount of debug and reputational damage in store for not very much gain.

If you are supporting multi device makers, and a normal audience, (ie people who don't know what CRDTs are) then you doing the hard work is far easier.

And thats before we get into ACLs for sharing.