Biggest problem is that it’s free. That means you’ve got very little bandwidth that’s usable since it’s being supplied out of generosity for no direct compensation that could be reinvested into the network. There’s just too many users and not enough bandwidth.
And because it actually works, it’s very difficult or impossible to police how it’s used. That means your precious bits are just as important as the 100,000 spam emails that another user is trying to send with the service.
Finally, you might not want to use it because you’re sharing the same exit nodes with many other users. This means services tend to block those IP addresses outright, limiting what you can use it for, and if you leak and identify such as your name maybe you don’t want that tied to an IP address that actual terrorists might have used.
I write this as someone who owns a bunch of official Tor merchandise.