I've been told...that piston activated rifles will not suppress as well as DI.

In a DI, the gas goes through the block and "relatively" wayyyy down the gas tube. Lotsa stuff happens during that time, mainly I think the barrel pressure drops quite a bit.

All piston-driven items, such as an FAL or (gasp) a Mini-14, draw the gas off the barrel, slam the piston back and vent out a gas port. That gas port is usually towards the end of the barrel, venting when the barrel pressure is still relatively high. Thus, alot of sound comes out the vent port.

I think it is somewhat similar to blowback pistols generally not suppressing as well as breech lock pistols, because the slide starts going back instantly, opening a "crack" in the chamber and venting at somewhat high pressure (noise).