MMORPG server

In Unity forum, there are a bunch of people discuss about this. Before doing anything, you should read through is it possible to make MMO with Unity first.

Here is one the discussion

Most people would give up on this subject.

If you want to pull this through, the only way to make a server authorative for Unity which is the most simple (still hard for most people) and not costly. Photon Bolt + Photon Server balancing is the way to go.

It will took around 1-3 years to develop a game that is playable if you have money to host server.

Edit: Aug, 2018 Other battle tested server solution is like:

Forge

SmartFox (mostly web game)

Unity about to rework Unet. You should expect about 3 different network solution for 3 main type of game. FPS, RPG, RTS.