Assault Grenadiers are perfectly fine. They only excel at very close range. Most people will use sprint to close the gap. If I'm not mistaken, sprint is not free (??). As USF get an AA HT to shut them down or any sort of vehicle as any faction really. Always try to be behind cover vs them because they absolutely positively need to be close to deal damage. And if they are constantly using nades and sprint, you know they will have no munitions for infantry upgrades or abilities.
They're also entirely defenceless vs any sort of vehicle, and don't have lategame utility abilities (Smoke, for example) like many other "Assault" infantry units. They're good at what they do, but they're in no way imbalanced.