Those 'unkillable' units really only result at the high end of the research when the defenses have outpaced what most other units can hit. Usually that is by being a step or two ahead of the World Standard. The issues I see are Tanks and Heavy Bomber.
For Tanks, they have 1 armor which gives them a 3.5 point advantage on defense vs offense. But what makes them really so much better is that the World Standard for tank defense is 8. To get Infantry or Arty or Tac Air that keeps pace with them would need to also be 8 Ground Attack. Unfortunately, Infantry Ground Attack WS is 6, Arty is 7, and Tac Air is only 5(!). But Tanks ALSO have a WS of 8 for Ground Attack...So, it behooves you to research Tank attack up because it will be cheaper and more effective.
Nothing else can have the defense that Tanks can for anywhere near the same cost. Again, WS for Tanks is 8 for Defense while Infantry only have a 6, and Arty, a 5.
So Tanks become the 'best' unit to focus your research on and the pay-off is that they are hard to kill except by anything but opposing researched tanks. But that strikes me as wrong because historically, many more tanks were lost to just about everything else BUT enemy tanks (tank vs tank actions werent all that common). Air support cant kill them and 'dug in' Infantry cant kill them so you are encouraged to research and build tanks of your own instead.
For Heavy Bombers, the situation is a little different. The have 1 armor, so they do get a die advantage there. There also have a higher durability (4). This means each point of Bomber Evasion is worth more than a point of Fighter or Flak AA attack. Couple that with the armor, and you need to basically exceed their defense to hit them rather than equal them. Fortunately, Hvy Bomber Evasion's WS is only 4, while Fighter AA attack's WS is 6, as is Flak's AA attack. So, this one is a lot more possible. You just cant be taken off guard as I was in my game. I still dont care for the Hvy Bombers to be quite that hard to take down, but at least you had to pay through the nose to research 2 points above World Standard. Note that if they DO make that investment, you also have to be 2 points above the WS to realistically take them down with Fighters and Flak, so if they pull it off, you are hurting unless you have equal resources to burn.
So, at any rate, I think it is the World Standards that have to be looked at, especially for Tanks. 8-8 is just so much better than anything the other combat arms can muster (6-6 Inf , 7-5 Arty). Bombers are probably OK, but I wouldnt mind seeing Flak/Fighter WS in AA attack go up by a point or else lower Bomber Evasion WS by a point to ensure that they need ESCORTS to survive, not just a very high defense stat.