I think your example is more a statement of the power of Stunning Assault than it is of the wording of CM's being strange or maneuver balance in general being off. Maneuvers cost a certain action to take, and within they state what you do as that bonus action, action or reaction. Also, they either directly reference the Attack Action, or they don't, which seems clear to me. Attack Action or 'Make an attack' seems to be balanced around being able to multi-attack for the exertion cost and the added effect of the maneuver. Stunning Assault has the problem of being relatively low level (2nd degree), and especially with maneuver specialization, relatively cheap for such a potentially powerful effect (3 or 2 points) while being extremely effective with two-weapon fighting. Most of the time, PB will be either 3 or 4 for characters that have this maneuver available, which results in somewhere between 2 to 4 uses of this manuever per short rest. For reference, both blinding maneuvers that I could find are 4th and 5th degree, and they either cost 1 exertion per attempt (4th degree stance), or 3 points total (5th degree action), with the low-level Eye Slash only allowing for a single attack (and thus a single save).
Now is Stunning Assault problematically strong? Perhaps. Considering 4 attacks at level 5 and 60% chance to hit you can force a creature to roll a Con save against your DC 2.4 times a round, probably for two or three rounds (unless high PB and/or Fighter), increasing to 5 attacks (or 3 saves) at level 11 for Fighters.
Con saves have traditionally been the worst saves to target in DnD, as that is on average the highest save monsters have. Of course, you will have an educated guess of a monster's Con save (frail wizard or hulking demon), so it will not always be so random, but on average, Con is not a great save to target.
Going off of a 60% chance to land the stun, in most non-Fighter two-weapon fighting use cases you will be able to stun enemies for a total of 2 to 6 rounds (assuming you switch targets after landing a succesful stun). With Fighter that number increases a bit, because they have more exertion and cheaper maneuvers. Is that a lot? Perhaps. But compared to decently powerful spells like Hypnotic Pattern/Forcecage/Maze? I am not so sure.
While I think it is reasonable to limit Stunning Assault to either heavy weapons only (excludes most two-weapon fighting shenanigans), or to only grant the stun chance on the attacks during the attack action (not during bonus action attacks or OA's), I don't think stunning strike ruins campaigns per se, and here is why. I have played in an O5E campaign with some decently high-leveled monk in the party. Stunning strike is awesome, but only if it lands, and it will not always against everything. Second of all, you are burning through exertion/ki points doing this. Sure, the hypothetical 16th level Fighter can use this ability a whopping 7 turns, but a lvl 8 Berserker can only do so for 2 rounds. Is the wizard boss absolutely doomed? If he gets hit 5 times, probably yes if he has no legendary saves anymore. If he has any defensive spells like shield or mirror image, it becomes slightly more unlikely, and if he wins initiative, your 16th Fighter better be watching out, because two can play the game of save or die. Try it against a boss-level paladin, good luck getting through his +14 (or so) Con saves (speaking from experience here ^^").
By the way, if you are the DM in this case and you find that Stunning Assault or other maneuvers cause the game's fun to be reduced: You are basically the boss. Rules as written you have the final say. Feel free to talk with players, reducing the power of these maneuvers slightly by not letting them interact with TWF for example. Or give them some nasty encounters to chew on, they'd better be permastunning the dragon while the rest of the party deals with his spawn, because as soon as that dragonbreath goes off twice, it's game over. Play into their strengths by basically requiring them to use it to the best of their ability 'or else'. Or alternatively give them a fight where endless con saves vs. stuns will not help them all that much (plenty enemies with sky-high con saves or even straight up stun immunity). Have lackies cast Lesser Restoration (well, rather a spell that actually will end the Stunned condition). Have the wizard somewhere out of direct reach of the melee goons. Make them work a bit to get to him, clear the field, and then after round 2 or 3 give them the guilty satisfaction of horribly dominating the poor old man with stuns he will not escape from. Don't always punish them with immune enemies, but also don't always go easy on them and let them use their preferred strategy either (which I would say is good encounter design anyways).