Pasting my reply from a similar inquiry on Facebook:
I suspect poly~ will still solve some problems that MC can not, simply because you have full control over the abstraction being duplicated. But, it’s more work to set up, and MC easily accomplishes much of what we build poly~ abstractions for in the first place.
I would assume it handles those tasks more efficiently, as well, since MC objects are fully compiled, where your poly~ work is still interpreted.