I would say, judging from the developer, Windward is one of the flagship games that he promotes TNet with. I'd say that he's is possibly one of the friendliest developers out there, helpful even if I ask dumb questions that he could just do a one-liner "RTFM" (trust me, been there done that).
TNet possibly would suit the bill for MMOs, because you can use it to be Peer to Peer. For example, I'm using TNet in my shooter project for clients to be saying "Hey, I'm here, here and here" which the server is just going "Okay, passing the packet to the others" rather than have the server do a simulation and have the client be a dumb terminal (as in, it feeds the server's player entity the values and "shows" the end result). For attacking and other things like explosions, on the server I'm planning to keep a X number of timestamps where everyone was, so when the player shoots, it goes "Hey server, I'm timestamp blah, I think I hit someone, can you check". Server then rolls back the clock and says "Yep, hit confirmed" or "Nope, missed". Ultimately, the server is a client, but it's running the combat logic.
There's other third party solutions out there, like Photon and SmartFox, but they are expensive subscription based or user based and have limits on how many messages (packets?) can be passed through at any given second. TNet is just a base model - the internals of a car - and it's up to you to mold your system on top of it.
To lock down TNet, just change the protocol ID in the code to something else. That way people can't just pull up Unity and a copy of TNet, guess your master server IP and go from there. TNet will stop connections with an error if protocol IDs mismatch (the TNServer will throw a "X has failed the verification step" message in console). You may also consider adding your own code (maybe magic packet that you must send to say you're X game client?) to detect people trying to spoil the party and break things.
