What sort of content do you say Tor onions can't host?

"Tor onion" just means that a server is (ideally) only reachable as an onion URL, which is only accessible via the Tor network. There is the limitation that Tor only handles TCP. Otherwise, one can route anything over Tor. In my experience, that includes HTTP(S), FTP, Tahoe-LAFS, SSH, RDP, Mumble, OpenVPN and tinc. And others, if I spent more time remembering what I've played with.