I've had it kill 4 in green cover. The thing has a huge range and is almost 100% unavoidable due to command lag. Whereas the guy throwing the molotov has to do some huge wind up baseball-type throw, the rifle nade just... shoots. And even if it only kills 2 guys, it knocks the health off the other guys to the point that they have to retreat or die. It's not OP, but it's annoying to play against, mainly due to the lag.
There is a very long wind up animation for rifle nades as well: One of the Grenadiers kneels down, puts the rifle butt on the earth and aims 45° into the air for 3-4 seconds.
Not talking balance on this, just wanted to mention it. Maybe you'll notice it in combat
And as far as I know the cover doesn't do anything against it because (most?) cover is directional. This means that it needs to be badly aimed for the cover to do anything.