Obviously smaller maximum sizes would require the rules surrounding horde bonusses to be reworked. As for them being useless. Of course they're going to something during the match, maybe they claim an objective, maybe they block a charge, maybe they score that 1 last wound on a big scary enemy. But you don't take em for that. The only reason you take em is for the shiny buffed up horde unit in the battalion. If you could get that buffed up horde unit without the two other squads you probably would swap em out immeadiatly. Mwha, covering the board in units rather negates the need for good maneuvers I guess. I'm mostly just talking about what I'd find fun though, not necesarly about if it works or not. Units of 40 feel so massive it makes the game feel clunky, units of 20 feel more appropriate. Maybe I just need a massivly bigger table
Sure, but somewhere in my list would almost certainly be one or two objective holding units that did nothing much other than sit around all game. If I can take them AND buff up a big unit in the process, that's a win.
Viewing it as a win is falling victim to the sunk cost fallacy. You're simply giving the tax-units you were forced to take to bring your fancy buffed up unit something vaguely usefull to do, but given the choice you'd have skipped the tax units and swapped them out with better or cheaper stuff.
One might argue that the buff on the one maxed out unit is worth the tax. You cannot just leave the tax units out. Me personally... I think it can be worth it. Depends a bit on the mission (and the rest of the list).
Sure, dealing with the tax units can be worth it if the star of the battalion is amazing, but they remain merely tax units they don't suddenly become "good" cuz they happen to give you acces to their powerfull friend. And in this case it is especially noticeable as the battalion consists of simply 3 times the same unit as now you don't even get a tax unit that does something wildly different from the main unit. To be honest, I would've loved if sunclaw host could be the following: 3 of any non-hero saurus units in whatever combination you want. In that case the tax units could be guard or Knights, both of which would actually add something beyond mere additional bodies to your army.