It's more their completely erratic AI coupled with their slow speed that makes them suck sometimes. When a unit targets 'anything' in this game that means it spends a great amount of time doing nothing. It's nowhere near as noticeable on heavy infantry due to their superior speed and numbers.
I love how they can easily blast through walls yet half the time they'll conveniently forget that and waste 20 seconds backtracking through a broken wall section for no reason. Or how they'll blast through a wall destroy one building out of five then all decide to back through the wall to chase one defender.
Anyway, I have max uni CWA heavy tanks and I think any sort of speed boost or internal trap negation would make them overpowered. I just like to grumble about things sometimes.
That said, if the hypothetical Space Age sees the same 15% offensive vs 25% defensive upgrades that atomic and CWA have featured heavy tanks will join the long list of of useless units. As the OP mentioned, they're already there when it comes to attacking most moderately upgraded atomic and CWA bases as it is.
What this game is in dire need of is a buildable unit that disables traps, instead of those only being available as card armies. In my wildest dreams that would mean doubling wall miners' space, costs and build times and making them true combat engineers. Perhaps that will reduce the 8 wall miner/40 motorcycle spam so prevalent in the game. Maybe it would make it worse. However, a completely new unit would be fine too. Just spitballin' here.