He is fantastic at turning around a gank IF his turrets are already set up and placed smart. Turrets both block projectiles (lee Q) and do damage. The mid laner should have to dive one turret just to attempt the gank, same with the jungler. Meanwhile you can run behind safety turret in the back to avoid skill shot death and pop your stun. They might even have to run back through your turrets. I've played heimer mid in gold games where im around 1/4 hp and enemy is half. Lee q out of the brush hits turret next to me and enemy laner charges in, I stun lee before he can hit and run behind back-turret, q-fan, run more. Mid laner turns around but doesn't quite escape the 1st turret (cuz they didnt attack it) and jungler is now 1/4th hp too. I push lane right after and barricade myself with turrets. Lee comes back and q to a turret, gets stunned again, explodes.
Its not that heimer is actually good or bad. People just dont know how to use him / fight him.
He is perfectly viable as a diamond champion counter-pick (not first pick) against melee tops or mids that cant wipe out the turrets with ease. Theres only a few he really does well against. Otherwise, he just holds his ground. That is, infact, what being a heimer is really about. You set the barrier, whether it be to stop a peel/skillshot, disengage, prevent ganks, or farming under your turret. Its just hard as hell to push on a heimer thats playing smart. I saw a heimer turret the river and behind wall before doing baron to successfully ward off contesting.