I've played several Tank Destroyers, SPGs and Light Tanks so far and reached Tier V with each of them. (Panzer IV, M41, StuG III)
My observation is that the overall difficulty level increases because you are faced with tanks which are more tough nuts to crack. In case of an SPG I had to learn how to properly aim at the weak spots and/or calculate with tank speed (I noticed that the higher the tier of the tank the opponent sits in the more aware they get of a possible SPG threat). I also get bigger guns (155mm howitzer in M41 for example)
In my Light/Medium tanks I had to learn how to use cover so I don't die instantly and how I can use my speed to work myself around bigger threats. I do feel that the higher tier I get my tanks become more capable.
In case of my tank destroyer StuG III I feel that this is not the case. I'm not as fast as a lighter tank, and I do not have the firepower of a heavy tank. (I can't shoot behind covers either). The StuG III has the same derp gun as the Hetzer but since it is a higher tier I often face Tigers and KV-1s. I also noticed that since the StuG III's armor is not as sloped as the Hetzer's I am more vulnerable. I don't feel the same progression with tiers as I do with other tank types. It seems to me that I did not learn something important about playing tank destroyers since the higher tier I get the less fun I have with them.
Do you have some guidelines for a puzzled tank destroyer player? StuG III specific answers are also welcome