Assault Grenadiers are sort-of meta.
According to commanders picked over the last month, mechanized assault commander stands at the 3rd most picked commander (3272 matches) for 1v1 matches. In 4v4, it doesn't even hit top 5 since assault grenadiers have shit scaling to late game even with VSL upgrade. Like mentioned, it's a filter unit since it more or less bashes in people who can't properly respond to assault troops. There's some glaring issues with assault grens that typically see them literally never picked in tournaments, but in pubs they're easy game enders. Off the top of my head (excluding the scaling issue I already mentioned):
1. No snares whatsoever means any vehicle counterplay will more or less go unpunished (USF AA halftrack rush, Soviet T-70 meta, UKF UC Bren carrier rush).
2. Costly to reinforce, which doesn't help when their RA (received accuracy) is not as elite as stock army options like Panzer Grens
3. Overshadowed by Panzergrens that have much better vets, superior RA and far better DPS drop off over range.
4. Assault troops can't poke or do dps safely like standard LMG grens, which means you always have to try to close the distance. If an opponent has a solid reaction, you're going to drop models like mad before closing, reducing the DPS of the squad which means you'll lose the engagement at that point.
I tend to find ass grens still pretty deadly though, even with their current position in the game. 15 dps per model at vet 3 is fucking insane (almost the same as British commandos), and sprint helps them get in the position to close the distance. Grenade volley is okay and against scrubs who don't dodge you are getting free wipes. All in all, great pub stomp option.