Grens are great damage dealers when vetted up. I think the biggest problem for them is their terrible early game power.
Yes and then lose that vet in the blink of an eye because 4 men lol. Ostheer's problem is facing superior squad sizes, whom lose perhaps 1/5 or 1/6 (or less if upgraded) of their fire power when losing a model. Lose a model as ostheer and you are down 25% in most cases (pgrens or stormtroopers), unless upgraded grens. However, with upgraded grens, you lose one model and are in a very dangerous situation, one good hit from anything can wipe that squad easily. Not to mention that retreating with only 2 models left is risky, as you can get wiped very easily. Compare this to to losing 2 or 3 models and retreating on a 5 or 6 model squad.
Then we get to the point where we have tier 4 with powerful units, which just don't work in most 1v1 scenarios. The needed investment to get 1 panther, which will then get swamped by everything else the enemy has build up in great quantity, is not worth it. You still need to crutch on tier 3. There is no way around it. However good stugs may be, fighting jacksons, fireflies or multiple mediums with a stug or 2 and a p4, is very frustrating, as your mediums get shutdown pretty easily by those long range tank destroyers, who have turrents mind you. Not to mention the easily accessible handheld at for infantry of other factions, such as bazookas and ptrs. Althoug ptrs and bazooka aren't the best handheld at, they do add up against tier 3 units pretty easily, especially when considering all the other stuff facing your tier 3 units.