Balancing a Momentum Mechanic
I can imagine this system working better if implemented as an additional layer to the general action system, rather than a standalone concept of managing actions - let me outline what I have in mind:

each action a unit can take has a cost
each player has an amount of action points that regenerate each turn (not all have to be used, but you can't save any for the next round). Depending on your game's nature, you may want that limit to be different between factions or similar distinguishing factors
there's a shared, infinite momentum range for both players that starts at 0 and never changes on it's own (such as, at the end of a player's turn)
players may (in addition to spending any amount of action points for any number of actions) shift the momentum bar towards their opponent for the amount of an action's cost that exceeds the action points of that player's current turn
they may only take a single action per turn that moves the momentum bar on their opponent's side, but as long as momentum is in their own favor, they may use as many actions as they'd like, spending momentum in addition to action points

Here's an example of how that could play out, using only movement as an example for simplicity:

Both players have 5 action points and control 5 Soldiers with a movement cost of 2. The momentum bar is at 3 in favor of player 2.
Player 1 moves 3 Soldiers by spending 5 action points and shift the momentum meter by 1 (to 4) in favor of player 2. Because the momentum is in the opponent's favor and player 1 has already used momentum this round, they may not use further momentum
Player 2 moves 4 Soldiers by spending 5 action points and shift the momentum bar by 2 for their third, and another 2 for their fourth Soldier, ending with the momentum bar at 1 in favor of player 2. Since the momentum bar is still in their favor, they could spend more points, but they decide to keep it this way to retain the edge over the opponent.

By making the momentum bar an isolated element, you're giving players a better chance to judge how their actions will shift the battle in their favor, as with your initial solution, the momentum bar would shift crazy amounts every turn - with this solution, it's an optional element that doesn't &quot;regenerate&quot; on it's own, and therefore represents a more fair, yet also unforgiving investment for players, as every action that doesn't pay off shifts the meter towards your opponent, and now they have the upper hand in deciding whether to save it for later, or spend it on a counter attack.
It makes it especially interesting if your unit action costs force players to spend momentum points or waste some of their action points, rewarding players for planning accurately. I can also imagine that certain units could have action costs so high that they could only be paid using momentum in the first place. By never being able to use momentum once per turn unless it's in your favor, you're not allowing any &quot;rushes&quot;, other than if your opponent has decided to give you that advantage due to their excessive use of momentum.
All in all, this sounds like an element to make battles less predictable in a non-random, tactical way.