(it seems entirely possible to me that Umenzi are "merely" tier one and not a problem faction)
If I can talk about this for a second, I want to say that as a player I'm okay with the fact that some armies are just going to be better than others. I think that is simply unavoidable in any rules system that tries to have a variety of armies. Some armies are just going to mesh well with the ruleset when the theme of the army is translated into rules. Bloodbowl is a game I play that has some real hardcore folks and which has had literally tens of thousands of games played (and tracked). Their rules team has accepted that there will be power tiers of teams, and they track them to make sure they aren't too far out of whack. For example, if a team wins 53% of the time it's a Tier 1 team, but they don't feel its so powerful as to be broken. Similarly, a team that wins 45% is certainly a very weak team but not something where they feel that have to address. While I can certainly quibble on the numbers (is 45% too low a win percentage? etc), I agree with the 'close enough' mentality.
This not to hand waive any and all problems, because if the problem is big enough it needs to be fixed. No am I saying I agree or disagree with Kevin on whether the Umenzi are in that category of "big enough problem that it needs addressing." I'm just saying for the record if an army wins, say, 53% of the time, that's close enough to balanced for me to be good enough.
Disallow units on "Follow" from being considered backup units.
From a rules perspective this would be a simple change. Under the Close command we would allow an objective of a friendly unit and would say that in that circumstance the unit will move towards being in a backup position of the objective unit, with the objective remaining until changed with a command action, the unit gets engaged or the objective unit is destroyed.
I'm a little confused here, because this seems to be 2 proposals. Am I correct in understanding that your two ideas are 1) limit backup to only Close and 2) include a "friendly" unit objective? If so, I have to wonder if it's be simpler to just go with #1? It seems to me that if you have a unit on Close behind your line, you can use the Indirect Path rule to get that unit into backup position pretty easily. Usually if I want to put a unit behind another unit, I can use close with a terrain objective that is near the unit it is backing up to get it there. That's not to say a "friendly unit" objective is a bad thing or that it'd hurt the game, I just don't know that its actually needed to solve that specific problem. Although maybe it'd be cool to have period.
But I am in agreement on removing Follow from the qualifications for back up. That's probably pretty good.
Limiting Setup Shenanigans
Sometimes one or both players engages in "shenanigans" that are essentially designed to work around the normal rules for starting the game. For example, in our game Kevin gave his line orders to close but with location objectives that were behind them. Literally the orders were, "OK, when the battle starts turn around, march a bit, then turn around again and march towards the enemy." In practical terms what he was saying was, "You're all on Close but we want to delay the engagement so wait around for a few turns. Oh, and you Chosen nudge forward a bit." He essentially got to keep his line on Hold and then Sound the Call without spending any command actions to do so.
I have mixed feelings about this. On the plus side it lets players come up with cool maneuvers, and even if they look goofy the net effect is generally not unreasonable. (There was an argument a while back that we should allow delay modifiers to orders so a unit could be told, "Close but not for two turns.") It's also something that can be vulnerable to a counter-maneuver, as we saw in that same battle. I was able to use the time and commitment of his units to adjust my own line and get a good matchup for my Stag Cavalry.
On the minus side, I can see this being a turnoff for some players, particularly someone new to the system. It slows things down, can feel like rules-lawyering or "legal cheating", and is otherwise kinda annoying.
I think limiting this sort of thing would be a bad idea on the whole, because 1) I don't think its actually all that strange, 2) I think it would hurt legitimate tactics, and 3) it's a rule that would affect everyone when you're really trying to limit 1 faction from egregious abuse.
First, I actually don't think its such a crazy, unheard of tactic. I know BGFW isn't history, but there are historical examples of this happening. For example, at Thermopylae the Spartans (and allies) didn't just stand at the mouth of the valley and get wailed on. They engaged in feigned retreat in the early stages to lure the Persians into the narrow valley. At which point they turned around and started the stabby-stabby. Alexander at Gaugamela had his army basically do a sine-curve where one side curled back on itself while the other flank came (way) round the corner.
Again, BGFW ain't a history simulation, and so I'm not trying to say that. I'm just saying that this notion that two armies just smashed into each other is a combination of Hollywood and Games Workshop (and a patronizing attitude that because medieval armies were untrained mobs all pre-industrial armies were all a bunch of untrained mobs), and frankly I think one of the selling points of this game is that it allows for very complex maneuvers. In fact, most people when I give them a demo think that between the simplicity of the orders in the demo and the small size of the table that this is just a no-brainer 'everybody crashes into everybody' game. That is, until after the demo when I explain to them how you can tweak and modify orders and get some very complex maneuvers out of the units in that same very small space. In fact, just last night I was talking to some of the historical guys who were watching me play BGFW and when I was maneuvering one of the things I said is "in this game,
like Field of Glory, you use the whole board, not just the length."
Second, I'm concerned that any tweaking will have negative impacts on legitimate uses of objectives. For example, last night I was playing Ancient Feud and the units we nominated were almost exactly opposite of each other. His was on the far left of his line and mine was in the middle (with 3 units between my grudge unit and the end of the line). I compensated by giving my 3 units to the left of my feud unit enemy objectives so they would go to the right to engage an enemy unit one-unit-over from where they started. Then, with a use of DC, I had my grudge unit go behind my line and run around to engage the other feuding unit on the far left flank. And also I had my longbowmen target his unit of axemen as, when combined with my greatswords, I felt i could create a breakthrough there.
I don't think that is really any shenanigans. Certainly the scenario forced it, but let's say due to deployment I wanted my 'feud' unit and the unit to its left to switch places, because the enemy opposite them were bad matchups. Combine with enemy unit objectives and ranged fire objectives, I don't think its that's egregious at all.
I remember when I was playtesting Alexander, I was playing Carthage and due to his use of Poor Man's Foresight, I had a unit of elephants on my right flank while he had overloaded his right flank with Foot Companions. I gave my elephants the orders to turned around and go all the way behind my line to a gap that I'd opened on the center-right. Meanwhile, I gave orders to the other left units to slide sideways and hold (eventually DCing them to curl back) while the right most units slide right to open up a spot for the elephants before completing a wide right hook. I gave terrain objective orders to at least 5-6 units to reorder my lines, and I don't think it was any shenanigans, but rather me trying to reorganize out of a bad deployment.
Third, as you said, this is mostly a concern for the Umenzi as in their case the spellcasting can make it more of a shenanigans thing. My first thought was to require spells to have LOS like indirect fire weapons (so if they turn around, they can't cast spells), but I imagine I'd just have the spell casters hang way back to keep LOS and give the front rank units room to come back. If that's the case, I'd say bite the bullet at errata the Umenzi spells. Give Hex a 7" range and maybe Bless a 3.5" range. Or maybe make the range of those spells equal to the Leadership Range of the Shamans/High Priests. Or maybe some other solution, but I think it'd need to be tailored to the Umenzi (be it an errata or in the rulebook).