It's true that it matters where the tank gets hit in terms of damage (more dmg taken from the back and sides) - but I don't know if this is relevant when you use the javelin type anti-tank rocket launcher (can't remember it's exact name rn), because it is guided and always hits from above
I agree that tanks are a team asset and shouldn't be easy for a single engineer to kill. But I also don't think that would be the case, if it took less than 6 javelin hits to kill one. It's not easy for an engineer to fire at a tank 3 times, without being shot by said tank himself. And I don't think a tank should be able to completely disregard positioning/cover because they can just tank every one of an engineer's javelin missiles, and still be healthy afterwards.
It's certainly a balance, not to have tanks be too easy or too hard to kill. In my personal experience from playing the game for 2 days now though, tanks seem to be wreaking havoc. I don't know if any of the later anti-tank missile launchers you unlock at later tanks do more damage though.