It does vs. HMGs outside of buildings, which is pretty common. There's also smoke, LOS blockers as you said... Really, there's a lot of situations where they're countering MGs.
There is a mechanic for them specifically for that issue.
Suppressed RETs will not shoot accurate nades.
If you are flanked, even CEs would hardcounter you.
Meanwhile, I'm not sure how you're supposed to micro LMG grens: they need to stay static to fire.
For example, by not sitting at max range and getting inside dead zone of RETs? They aren't going to do much with these carbines of theirs.
One is a muni-heavy (early game), single round, 'skill-shot', the other is a single-time muni cost, auto-firing weapon requiring no user input outside of being 'in-range'. They're not comparable.
And both are projectiles that go over LOS blockers.
Mechanically they are exact the same.
Balance of them is not comparable as they serve 2 different purposes.
In fact, all grenades behave that way, regardless how you launch them.