What is unfortunate is that a lot of these poor units are iconic Cold War weapons, that never see the light of the day in this game. A lot of these are simply victims of double-standard modelling, where a unit with similar stats in real life has far better stats in the game.
This is just my opinion, it is a subjective list. At this point I don't think Eugen will re-role but I might as well write my thoughts down...
1. F-4 Phantom Series
The backbone of numerous western air forces until the arrival of the F-15 and F-16, it excelled in the roles of air superiority and ground attack. In the hands of American, Israeli, and Iranian aviators, it proved itself in some of the most intense fighting in the cold war.
Sure, the American F-4 makes an impressive blanket of napalm and the German F-4F KWS is a decent AMRAAM carrier. The Wild Weasel is OK. But the rest of the F-4s in game carry old Mavericks, shitty sparrows and mediocre bombs and cost far too much. To add insult to injury, the UK does not get a single F-4 phantom despite the fact that they used hundreds of them, even upgrading them to carry the SkyFlash.
2. M60 series
It's a Leopard, but worse. Much worse. ¯\_(ツ)_/¯
The M60A1 ERA on paper looks to be a fair match for the Leopard 1A5, but the loss of that 175 m range, really, really hurts. Now that NORAD is a coalition, the last nail in the M60's coffin has been hammered in. The disadvantage is small but the result is that this tank is HARDLY EVER TAKEN.
I'll post more later if I can write anymore, but I'd like to see your opinions as well. What units/unit lines could have shined in game, but are simply undermodelled into uselessness?