Yes. I host my own nextcloud, I don't need their calendar. But that also means I don't need their drive. I only need the VPN and the mail and simplelogin is a nice bonus.

Use it but don't rely on it. Celeste uses rclone. The rclone support was temporarily disabled from Proton's end a while back and also, the rclone backend still has a bunch of bugs and the developer seems to have gone missing

They're referring to the quality of integrations with third-party systems, like the built-in CalDAV support basically every OS has. For some people, using just the calendar app is fine, but others want that deeper integration so they don't have to rely entirely on Proton to provide features in their frontends that OS apps might already handle.

For example, on Android I might want to let other apps access information from my calendar (e.g. my launcher so it can show me events from within its built-in schedule widget). Same goes for my Thunderbird client on Linux, it'd be nice to have the calendar events be integrated there too. Unfortunately, they currently only support a mail bridge, but the official Proton account on Reddit has made a few comments stating that they're "looking into" adding CalDAV support to Bridge, but there's no official timeline on when or if that'll actually happen. I'm willing to bet it eventually will, but I'll say I'd definitely appreciate it if they did.

It doesn't integrate with android or linux. You are vendor locked in. You can only use proton's app. Usually carddav and caldav go together, my tasks (and now kanban board. thank you jtx) and my calendar are very well integrated.