> why would you implement an Anubis PoW MITM Proxy, when you could just simply block on UA?

That's in fact what I was asking: I've only seen traffic from these kind of companies and I've easily blocked them without an annoying PoW scheme.

I have yet to see any of these bad actors and I'm interested in knowing who they actually are.

Huawei. Be happy that you haven't been hit by them yet.