Yeah, single units won't work. It is builds that _could_ work.
So certain combinations of units. Like this:
The following unit combinations are not allowed in lists for this tournament (Summoning included):
- More than 6 Kurnoth Hunters in a list
- More than 9 Tzaangor Skyfires in a list
- Kunnin' Rukk bataillon with more than 20 archers in a list
- Lord Kroak + Astrolith Bearer + Balewind Vortex in a list
- More than two Wight Kings in a list
- More than two giant Squigs (Mangler Squigs or Forgeworld Colossal Squigs) in a list
- More than one Mourngul in a list
...whatever. Such stuff. I am pretty sure it is possible to describe those builds in a few words. It would include, say, the key characteristics of the top 3 or top 5 placed armies of the last four big (official or something, there has to be some kind of "main tour" of events) tournaments. So a maximum of around 12-20 "locked" builds.
About Bataillons: I think there is not one bataillon that is super broken, it mostly is a combination of stuff that might be broken.
Kunnin' Rukk with melee Orruks is fine, but not with a big amount of archers.
Single unit spam can be a problem if units are undercosted, but doesn't have to be. Skyfires and Kurnoth Hunters aren't broken, but playing a list with 15 Kurnoth Hunters probably is.
And keep in mind, this isn't necessarily about broken lists, just about strong ones. If they did well they are not allowed for the next four tournaments so people have to choose others. If that kicks out broken lists that is just a positive side effect. You would recognize a really really OP list by it winning (placing in the top 3 or top 5) exactly every fifth tournament.
Would it work? No clue. Perhaps. Perhaps not. I'd like to try it and see but tournament hosts probably won't do it to avoid possible backlash by the competitive community.