> On another note, won't this fail when there are other references to these arrays?

Yes, this is something I've been considering. As long as `fit` doesn't attempt to reuse an existing Tree, and we have no `partial_fit` or warm start, there shouldn't be other references until after building is complete (which is the where the previous model failed). 

Given this, I should probably add `refcheck=False`. Had we need to support `partial_fit`, I assume we could handle the error by creating an array from scratch.