Yea I guess the fact that they don't have upgrades is a double edged sword. As you've said a lvl5 regular arty would deal about the same damage but that's only at day 12 + 1 day of research so you'd have a 6 day window where in damage output, you're at a big advantage. Lvl5 is also the max lvl for arty discounting elite lvl and even then it doesn't really one up the rrg, as you've said it only reaches comparable damage values. Lastly, in a rrg v arty situation, the rrg would probably dish out 2 attack ticks by the time the arty stack gets in range so there's also that.
Against air, I don't really see this as an issue, artys themselves are vulnerable to air but with AA's or Ints it's manageable. I don't see how that can't be applied to rrgs as well. The idea behind the militia is cheap meat shield to bulk up the stack, a lvl3 militia has the same hp as a lvl4 inf, if you unlock their elite lvl their hp jumps by another 50% about the same to that of a lvl6 inf. Yes they're slow but rrgs are slower so it's not like they'd slow down the stack.
On speed, I'll concede it is an issue. It really is a snail of a unit. It's almost amazing how even with great damage values and the biggest range in the game, it's speed just makes it so repulsive, so repulsive that it trumps whatever good it has to offer.
Against air, I don't really see this as an issue, artys themselves are vulnerable to air but with AA's or Ints it's manageable. I don't see how that can't be applied to rrgs as well. The idea behind the militia is cheap meat shield to bulk up the stack, a lvl3 militia has the same hp as a lvl4 inf, if you unlock their elite lvl their hp jumps by another 50% about the same to that of a lvl6 inf. Yes they're slow but rrgs are slower so it's not like they'd slow down the stack.
On speed, I'll concede it is an issue. It really is a snail of a unit. It's almost amazing how even with great damage values and the biggest range in the game, it's speed just makes it so repulsive, so repulsive that it trumps whatever good it has to offer.