It depends. Some services are much more suitable than others.
Some services are 1:1 ratio. That is, uploading a file results in a download that only works once. So that makes them rubbish for malware, you have to be spear phishing somebody and even then it buys you less than using Tor would.
Some services are only encrypted in transit. So bad guys can't intercept or alter the data, but at rest on your server it can be scanned for malware, copyright infringement, whatever the provider wants to scan for.
Some services cost money to use which is an obstacle to bad guys who most likely want more money and not to be paying money up front first.
Firefox Send was encrypted in situ (the keys live only in clients, so the server doesn't know your keys), it was free to use, and it allowed either unlimited or very large ratios.
So that makes it potentially very attractive. On top of which, it has this nice trustworthy Firefox name. Grandma Jenny's kids have told her not to go around installing stuff from just anywhere, but they did tell her _Firefox_ is trustworthy after she got flustered when it auto-updated. How is Jenny supposed to understand that this link to Firefox Send isn't Firefox?
