The Panzer comes so late it needs to be more useful to gain an edge off of it. It's why the Zero is so very mocked here, and the B17 is so divisive. They are very, very late-game units that are only slightly more useful than their non-unique counterparts. It is a basic rule of strategy - the later it comes, the more advantageous it has to be for it to be worthwhile. It's why Rome and Greece and Assyria and the Huns and the Zulu are so devastating. Their uniques are simple, but they come at a time when that simple advantage is a huge bonus, since it snowballs later in the game. Capturing a capital in 1500 BC yields far more strategic advantage over time than capturing it in 1962 AD, since you can utilize and build up that city far more significantly at a time when it is weaker and your opponents don't have much better they could do, and with far less resources too.
And honestly, some of those civs get more of an advantage anyway if their same bonuses were applied to the Atomic Era. +1 movement? How sweet, that compares so very favorably to Impis getting a dual attack and having all the Ikanda bonuses by that point, which include +1 movement as well. And really, having a movement bonus on an armor unit is only a tiny little difference, since the best uses I've found for mounted/armor is city sniping and getting rid of ranged/infantry units, both things that one can do handily with 4 movement. It's handy at times but hardly anything you can base an entire strategy on if you're really warring all the way into the Atomic era, whereas a lot of early-game stuff you CAN base a strategy upon. That simple increased combat strength of the Ballista means it can take out those fortified cities way more handily, when Catapults would otherwise do somewhat marginal damage, and the Legions can hold their ground way longer than Swordsmen, while your rear lines can build a roadway out and get city connections up in no time.
