The 2.x driver is kind of a mess, IMO. It is very clearly designed to serve Mongoid and it's pretty obvious that only web apps were considered when writing it (you can only talk to one database per client instance, it has a single global logger that doesn't permit separate logging levels per client/database), and as a result it breaks a lot of the conventions from the 1.x driver that would have substantial implications for MongoMapper. It's also fairly unstable (I've run into thread cross-talk bugs) and incomplete (no ability to set cursor flags, for example, really awful logging mechanisms); I've done a few small projects with it, and I've had to use my own forked version just to get parity with 1.x features. It's improving, but right now, my personal opinion is that it's a pretty substantial regression from the 1.x series driver. I imagine that rebuilding MM against it would be quite painful. It's also, curiously, extremely slow for me under MRI, though JRuby performance is pretty decent. I haven't yet dug into why.
I'd like to do a new major version of MM for Rails 5 and mongo-ruby 2.x eventually, but that is a huge undertaking which I don't have any time for at the moment, personally. It's something I've started to poke at a few times, but it's a deep project.