I'm not disputing that it's harder to counter demo, my point is that the loss is of much greater consequence if you don't happen to sweep the teller, which I think offsets that difference.
This leaves the much more relevant issue, which is that both can instant delete full health units. I was a bigger fan of the model capping for demos, and I think the parallel change for the teller would've been nerfing it's damage slightly.
Demos are currently useless, is anyone going to tell me that the teller will become useless if it merely reduces a t70 to 20hp rather than killing it?
I wouldn't say that is the case. First, even if you only kill single infantry squads with your demos, there are still squads that cost around the same as most expensive LVs. And cheapest LVs also cost similar resources to cheapest infantry squads. Which means the two losses are comparable. Secondly, it was much more common to see two or more squads killed or damaged by a demo than it is in the case of two vehicles. It is simply much easier to pull of. What is more, you can pretty much kill everything you can with a teller with demo, as long as you can make sure your opponent won't spot it. And finally, back in the days you could also increase the damage output of your demo by igniting it without using the detonator, for example by putting a mine under it. All in all, as long as scouting is provided, the old demo offered everything that teller could and more.
As for the solutions. I myself am not a fan of model cap. It didn't work in the case of soviet/universal mines. Why? For two reasons. First is that killing up to 2 models means that with cheaper squads you can actually gain advantage by demining the field with the lifes of your troops. The best example here is ostruppen who are pretty much always wprth less than 30 muni plus the engineer time if the mine is in a good spot. The second problem is that coh2 is all about risk management. Even if soviet mine would deal fatal damage of two models on average, but still had a chance to kill squad, then it would enforce good play forcing players to be more cautios and use sweepers. The damage value would be similar, but the risk would be much greater. As it is now, the risk of stepping on a mine is so small that the mine would be valued more if it could only be detonated by vehicles, couse only then it is worth the effort and cost to place.
Coming back to teller, there are ostheer T1 strategies, that use teller bait as their only hard counter to LVs in the early minutes. They are not easy to pull off, so I wouldn't call them OP, but they are nice to use just for the sake of the number different working approaches that ostheer faction gives to the player. Nerfing the teller would kill these and we need more depth and different working strategies, not less. In my opinion, the teller is expensive enough for as munition starved of a faction as ostheer is, to be well suited for its damage. To place one you have delay medical bunker, flamer, first lmg or save 2 fausts. That is a lot of different early disadvantages you have to agree on just to place a teller in the early game. And you aren't even sure it is going work till you make at least 2.
Btw. The one shot kill abilities are overrated in general. As long as there is valid counterplay that makes it certain your unit is safe, it is a valid ambush tactic and play who loses a squad can only blame himself for beeing too greedy.