Never underestimate infantry. 2 full groups of regular infantry (not plex) can knock out pretty much everything short of specific anti infantry vehicles and air speeders if you micro them right. If you put them in a bunker, they are even more terrifying. They're cheap and expendable so if you do it right the gains you get from them relative to their cost is insane. If you take some time to learn their ranges and how to move them around efficiently you can wipe the floor with your enemy.
As for the Jedi, I use them sparingly, kinda like the heroes. To me they're pretty much just for killing mobs of infantry, then getting out and healing, when I don't have defenses set up to deal with them. I don't think they're really great against vehicles for the most part imo, so if there's an enemy planet loaded with infantry and barracks a few Jedi can save you some heavier units that would get overwhelmed by the mobs.
Moral of the story is, every unit in the game is useful if you know how to use them right