It’s not that it isn’t recomended, it’s just in the category where you can have extra configurability which the other providers might not provide.
Not really, their privacy policy does state they don’t log or sell any data. When you create an account there is a user-facing logging feature which you might want to disable, (it’s meant so a user can trace what requests are coming from their network) but it can be easily disabled. This feature doesn’t apply to Firefox’s native DoH support though as that doesn’t have an account by default.
We reached out to them for clarification on their privacy policy though.