Directly comparing mainline infantry is useless. In context grens are probably the easiest mainline inf to use. But it's not like Rifles or Cons can't be used to great effect. And once your opponent knows what he's doing with Rifles and Cons LMG Grens suddenly feel a lot weaker. I would say in a game where both sides play sloppily (aka 99.9 percent of games played) LMG grens are superior cuz they don't require much decision making. Playing them is basically purely reactive. You attack move. If your opponent doesn't close in you just keep attack moving. If he does close in you retreat once he's close.
Playing Cons on the other hand requires clever stalling (spam sandbags at every opportunity) and maximum usage of utility like merge and trip wires.
Playing Rifles is even more difficult as they tend to bleed a lot. There's barely any player actively playing US who knows how to do clean and reliable rifle play (Inca probably being the only one coming close). This is because most top players neglect US and US specialists like Dave and DevM have left the game. Anyone who has been demolished by Dave should know Rifle's true potential in wrecking Grens imo. As of right now most high level US players seem to rely on knock out pushes that can be countered by appropriate reaction from the axis player.
Rifles take a big turd atop of grens, volks and fussies. The only problem is, most 3v3+ (and plenty of 2v2) maps don't really have proper sight blockers for the rifles to close in. It's mostly medium-long range cover to cover fighting, where volks and grens will win. And once the lategame comes, sight blockers get torn down and it's basically obers/grens/fussies blobs that dominate the field. BAR rifles are great close-medium range, but truth be told, you won't have a lot of close-mid range firefights that last longer than 1 second, come late-game.
Another thing is, USF doesn't have a brummbar or anything like that that can put a ton of hurt on blobs, safely. Against soviets, it's tricky to blob, as you can get wrecked by a good katy barrage. Vs USF and UKF, you can easily; especially since the calliope nerf which put the final nail in the coffin (extremely risky to use) of that commander. Now everyone plays airborne, which can give you that long range firepower + (a big plus) sight to spot those wide-ass-arc MG42/34s, which, on most maps, can lock down entire sectors.
Grens definitely do not deserve a nerf, but rifles should get a bit more DPS on medium range (just medium). Brits, I won't comment as the majorly-major majority of teamgame brit players are worse than a retarded headless chicken that got fu**ed by a big black cock. Soviet cons are pretty much perfect (also, majority of soviet teammode players that go for penal builds are extremely bad and inefficient, seldom are players that can pull off a penal start in 3v3+)