Generally, compatability:

No SMTP (so I can send emails automatically from my servers for logs etc), no calendar sync, email sync is only hacky with the bridge, android does not have email sync at all, no contacts sync.

The calendar does not show the birthdays of contacts.

Proton drive for me is useless because no GNU/Linux client (if it comes, pls just make it something I can run headless).

There is more, but that’s what I find most annoying. If I have to chose one of these then I’d say that the groupware stuff needs to be synced to any client easily (includes calendar and contacts IMO)