The thing about hard caps is that they artificially restrict builds. Build diversity has recently been increasing, in my opinion, and thats a good thing. I like being able to puruse alternate build paths and imagine other people do too. Thats why I push for units to have as low of a cp as possible (or be tied to tech), and 0 cp callins to not start on cooldown. Coming up with a cool build you would like to try and having the game tell you "you're straight up not allowed to do that" sucks. Spammy builds should be prevented through good design and meaningful weaknesses (...and good number tuning, which can take a few tries...), not artificial hardcaps.
As for heavy tanks, the hard cap in current coh2 has likely stayed out of both principle and a fear of creating another callin meta. If heavy tanks were tied to teching, I would imagine hard caps could be done away with and the vehicles rebalanced.
Yet the game has a pace and that should not be upset by dropping units that are simply from another time frame.
In the February 18th, 2014 Relic had to revert the change of Shock troops/Guards(/DShk/M-42/120mm) from 1 to 2 simply because these unit although weaker than now completely dominated the field.
So did assault grenadiers on release and they had artificially toned down "Dispatch Cost from 280 manpower to 200 manpower and 50 munitions", "Assault Grenadier dispatch cost from 200 MP and 50 MU to 320 MP" and then simply nerfed.
So did Osstruppen who are now on cooldown.
Unit available before minute one should be balanced vs stock units available that time (mainline infantry) or quickly loose power like assault grenadiers. If these call-in unit need to scale better than they should have weapon upgrades available to them gated behind tech progression.