Anything can be defeated, depending on what you have access to.
As psavola said, the reinforcements are very finite, with 'only' two waves. But I've seen more than one optimistic streamer get creamed by these fellows.
The biggest issue is that you either need to kill the first wave within five turns, or have something that can take out a tank. Given the trouble people seem to have with Red Dawn HQ with just armoured cars, that's much easier said than done.
Edit to the edit: I've sometimes done that. But by the time you can do so easily, a few BO guns and amnesiacs in black suits aren't quite the prize they'd be earlier on. Maybe if you lack MiBs to research.