If you have issues with getting gas in your face when using a DI gun with a suppressor, just switch to a heavier buffer. On the other hand, a suppressor is usually quieter on a DI system because with a piston there are hot, loud gases being vented near the gas port.
True story. A H3 buffer does wonders to eliminate gas in the face, and a full auto bolt carrier and better charging handle will help as well (BCM Gunfighter or PRI Gasbuster for example). And for those who really want to address the gas concern, there are plenty of adjustable gas block options out there that will allow you to tune the gas on a DI system. So I'm not sure that a piston gun has any distinct advantage here.
I'll add too that as an 07/02 FFL who shoots lot of full auto guns, I'm not sure a piston gun has much advantage over a DI even in sustained automatic fire. The kind of sustained fire required to heat internal components to the point of damage that some have suggested is going to wear out other parts of the system anyway, so that if you're really dumping mags at that rate of fire you're going to run into other problems beyond the BCG. We've seen for example barrels shot out and fire control groups broken, either of which can and do happen on piston guns as well. One of our AK's developed such a problem with trigger pins walking out after time that we had to finally weld it in place, a problem that came about directly as a result of sustained automatic fire.
The point is that automatic fire puts a level of stress on the entire weapon system that is going to cause unique problems regardless of whether it's a piston or DI system. Knowing that, we prefer to run DI guns since access to parts and ability to repair them is much easier and cheaper than any of the piston systems.