Big surprise but there are other modes as well. Which also happen to be more popular. Essen Steelworks, Ettelbruck Station, La Gleize, Angermunde, Lorch Assault, Lienne Forrest are all (semi)urban maps on which Feuersturm shines. Not every doctrine or unit needs to be (or can be) viable across all modes and that's fine.
I mainly play the bigger teamgame modes and Hetzer rush can definitely be a very strong strat at higher levels, but only if you are far ahead in the fuel department compared to your enemy. Otherwise it's just not worth the risk compared to waiting for a P4. Getting caught off-guard with hetzer by a medium allies tank or getting your raketen(s) wiped by a lucky mortar/nade/tank shot etc is a very big risk and puts you so far behind if it happens and you got no P4