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.
What you're describing is a Tor hidden service. Hidden services are separate from the Tor relay network itself, which is what I thought you were referring to as "Tor onions".
Hidden services are optimized for confidentiality over performance. Using them for bulk data storage would place a lot of load on the relay network, and it's not clear what security problem this arrangement would solve.
As far as I know, "hidden service" is deprecated, with "onion service" the current term. And it does tend to get shortened to "onion". But I admit that it was confusing. Because relays used to be called "onion routers". Which is also more or less deprecated, I think.
The security problem is Stores being physically located and compromised, based on IP addresses found in traffic logs.