I've been using ESS Profile NVGs since about 2005 or 2006, never had an issue or incident. Then again, I have a fairly broad cranium and I have RX inserts (polycarb lens) under them, so there's no real gap there even if something slipped in from a weird angle. But I could see the gap being present on someone with a narrower face.
Used with a helmet (TC2000) there's no angle a BB could slip under the lens unless you were standing on the person and they were shooting straight up or someone stuck the muzzle of their gun under the side of the helmet.
So like a pair of jeans you kinda have to try them on and see if they fit you properly.