Hi Philip,
While that's probably the most compatible choice philosophically, A2 uses MongoDB pretty aggressively. One can get direct access to the collections and they are expected to work like they work in MongoDB. We also use the full text search features of MongoDB.
In A2 0.6 (the "unstable" branch of the apostrophe module), the code is much better factored and it's much easier to figure out where collections are being created, but the expectation is still that you're providing something 100% compatible with MongoDB.