It's not the exact interaction that makes it (in my opinion) unbalanced. It is what that interaction allows. A lot of abilities interact with the attack action, like two-weapon fighting, flurry of blows, Press the attack, shield focus, Daring charge, and Commanding Presence etc. The list just goes on. The list goes on. Other abilities interact with other actions some maneuvers use like do, like dodge and some shields. If the maneuvers have a self-contained action then these abilities don't trigger, but they use the actions as described which means they do. This creates all sorts of interactions that are either encounter ruining strong, or class dwarfingly powerful.I don't want to necessarily hold up the maneuver system as a paragon of balance, but I'm not sure in what sense you mean unbalanced. You seem to be comparing inputs, instead of the actual resulting turns. Does it matter if the fighter takes the Dangerous Strikes action, then another attack action, or if instead dangerous strikes was written as a once per round free action that modified all attacks for the round?
An active example from the game I am GMing. :
My fighter and my barbarian (double sword exotic weapon), have both realized that two weapon fighting is way, way more effective than every other option, not because of damage, though that is more too, but because of maneuvers that can force save after save until the boss fails and is effected. Even legendary creatures become stunned, or blind or something after 4-5 saves in one turn. Now the fighter, who has stunning assault has made being an adept unappealing because they can't keep up with his 5 stuns for 2 exertion (maneuver master). And both than move through the spaces of the enemies and attack the caster in the back with 4-5 attacks. Even my casters are struggling to keep up with mob control as they can usually affect only a few guys without risking affecting party members.
Edit: I suppose what I mean to say is that balance-wise, in my experience so far, maneuvers should be discrete abilities that don't interact with other abilities.