Command panther and Pz4 aren't limited to one? Ostheer Puma is probably the exception confirming the rule
Mobile defense is literally only used because of its call-ins. It is the only thing that makes it strong.
And limiting them command variants to one is because of their auras. Lendlease wasn't limited to one, kv1s weren't limited, ect;
If anything the rule would be heavies and aura units, in that case the puma should be tied to tech. As right now it seems completely arbitrary.
And the point still stands that not waiting for the unit to build and instantly having it hit the field at a certain resource threshold will make really unfortunate engagement timings late game and can completely win games for people. Which has happened twice just the last two weeks alone. Obviously if the units are tied to tech they should have their costs lowered to reflect the cost to tech to them, but I'm not sure that really fixes the issues with heavies either being ultra effective or completely worthless.