Agreed if there were a lot of exceptions that might make sense, but I’m not sure the complication is worth it given the examples you list.
I also don’t see why a dedicated shooting unit should have as many melee attacks as a dedicated melee unit, it doesn’t seem a given to me. The melee unit would likely have some extra command to give it more attacks or static factors if it was warhammer for example, which is abstracted into the profile.
And 5+ with 20 attacks is 6.66 hits, not 4? And those hits have a better wounding chance than the ironclad hits and at range. So against armour 4, they do 4.4 wounds at 24", while the ironclads do 6.3 in melee, with the difference reducing as the defence increases (both do just over 2 wounds against defence 6). Not so great a difference. The Ironwatch will also still do 3.33 wounds in melee.
I’m not saying the ironwatch are the best option, I agree they are overcosted somewhat and a slight reduction in points seems the best solution to me. But my mate uses two hordes of them (one of x-bow, one of rifles) pretty well. Combined with a few war machines and warsmith it can make a mess of high defence opponents, and if you commit enough to kill it you risk being overwhelmed by four more hordes of melee troops. And looking at damage output is only one factor to consider with these too. It’s also a big bunch of unit strength that needs quite a lot of resources to actually kill.