The biggest problem really is that in a lot of cases you want your tank to be able to dish out some damage as well, so "classic" heavies won't be used too much. A second problem we have not bumped into much yet, but that does appear is the fact that heavies tank multiple units. If i bring up that last magic city map again, if i had put Deine in the middle to tank the first big golem, that'd be fine for that golem, but she would have eventually tanked both golems at once, killing her fast. Being able to tank more than one unit is often rather bad for endgame tanks since they do not want to take on the "boss" AND their underlings.

Heavies pretty much only exist to block tons of units, but in a lot of cases it's better to just kill said units faster with some unit that doesn't require their skill to deal good damage.