I would not suggest to get 3dGS just for the multiplayer capabilities,
these are not really the "killer" features.
But the rest of the development ist quicker than in other engines.

For a MMO I would suggest to get a decent plugin coder, who can get
around the limitations of the 3dGS multiplayer.

You can "somehow" set up a multiserver system, but
you have to work with a lot of workarounds to make it a serious game.

And small multiplayer-shooters (that can be made with native-3dgs, shure)
are not a realy good example for a MMO game, as it requires other methods.