I think the caps a great Idea and I agree with all you've said except that of warding being useless. The problem is in reining in the assassin to a point that it is no longer THE super stack he would be boring and useless and wouldn't do his job taking out super stacks.
Questions are -Which should be stronger 8 on 8 an Assassin stack or a super stack with ALL other bonuses?
Has ambush become mandatory as part of a super stack?
Why should I even build 4 or 5 turn units if they won't last?
Has it all it come down to in super stack battles is who's got the highest ambush numbers and who rolls first- that sounds incredibly dull. Wheres the strategy?
Some of the more clever players I've played have at the right times utilized what I considered a relatively useless skill or an unconventional strategy and taught me harsh lessons, and I'm glad they have the options for creativity. I think a warding amulet or some such Item in a lv 3 ruin would be a sweet find myself. I don't think it should come down to a guess what I'm building scenario, but having an option to level up warding on your 2nd hero after your first had a nasty meeting with a high level assassin would be a nice option, far from useless. I know many people have particular preferences about map size etc and the "overwhelm with numbers" strategy isn't going to play out the same on all maps such as those with less cities/smaller maps. And even with the cap at 70 people will just use slightly tougher fodder, or pair one up with a dragon, and they'll still be overwhelming. not a complete solution.
So.. if that is all dismissed in the name of simplicity and you wanted to keep to one skill I still think ambush could successfully be used to cancel itself out just like all the other skills currently do. If two opposing stacks with a Pegasi cancel each other out etc. etc. Two equal assassins should cancel each other out too leaving no ambush, as they are both equally quick and sneaky, ambush should be calculated on the difference.
I've taken or lost too many costly investments of planning, time and money that poof into thin air because my/their high level assassin with crows comes along, but I would hate too see another unit nerfed to become undesirable. I think Steve or whoever knew what they were doing when they add warding, keeping an exciting and usefull skill and yet providing a way to keep it under control. I would be so in favor of the cap AND of some kind of insurance, and I'm not alone as it has been brought up a number of times. If that happened, we might rejuvenate demon usage, who knows?