I came back to add: One of the things that just popped into my mind that Calcite has that we don't is query planning. Obviously since MQL was developed in house we didn't get any optimizations out of the box. We've since added a layer that processes the parse trees looking for optimizations and while it isn't quite query planning it has yielded some significant performance gains (or rather poor performance mitigations, haha).

I'd imagine getting this for free from Calcite would have been really nice.