In the case of the four. Must Parry, Stun, Stun Must Parry, and Stun Unable to Parry, Must Parry is the better of the four.
Must parry # rounds—(e.g. “must parry 2 rounds”)
For this number (#) of rounds, the target may not attack; they may only parry with half of their OB, and the only other actions allowed are movement and maneuvers, both modified by -25.
Stunned # rounds—(e.g. “stunned 4 rounds”)
For this number (#) of rounds, the target may not attack, they may only parry with half of their Offensive Bonus (Normal DB due to Quickness, shield, etc. is not affected), and the only other actions allowed are movement and maneuvers, both modified by -50.
Stunned and unable to parry # rounds (Stunned-No Parry)—(e.g. “stunned no parry 3 rounds”)
For this number (#) of rounds, the target may not attack, they may not parry (Normal DB due to Quickness, shield, etc. is not affected), and the only other actions allowed are movement and maneuvers, both modified by -75.
In the case of the others, being Stunned removes actions you can take and reduces the amount of and type of actions you are allowed. Though you can take a Stunned Maneuver check, even if untrained at -25, a failure still leaves you Stunned.
Must Parry only affects the attack round. All other options are still open to you, all be it at a -25. Movement and Maneuvers cover a lot of ground.
Maneuver: An action performed by a character that requires unusual concentration, concentration under pressure, or a risk (e.g. climbing a rope,
balancing on a ledge, picking a lock, etc.). Maneuvers requiring movement are “Moving Maneuvers” (MM), and other maneuvers are called “Static
Maneuvers” (SM).
Static Action (Static Maneuver): An action performed by a character that requires unusual concentration, or thought under pressure, and does not involve pronounced physical movement.
In the case of Static Actions, it has been argued to me successfully, that casting or activating magic items count as Static Actions. Still on the rope on that one, but the arguments given were compelling. So I let it go, provided I can apply it to the enemy as well.
Now I am not saying that a character who is also a caster that receives a Must Parry result, could just cast a spell at -25. But there is a lot of room for other things that don't involve attacking.
Also understand, that Must Parry takes zero amount of your action (not counting the -25). It is an automatic function of your DB. The fact that you are being allowed to apply any part of your OB to defense is not a penalty. The only thing you are being denied is the Full Parry function.
Granted if all your character does is "Swing the Axe" then stand around until they can "Swing the Axe" again. Then maybe it's more an issue of other skills they are lacking.
Take the time to move, look around, take an herb or potion to heal, change weapons, reload a weapon, prep a spell, activate an item, etc.
Must Parry is a function. Not necessarily blocking their attack with your own weapon. Among other things, it's dodging, ducking, dipping, diving, and dodging. This is especially true if hit from range or as a result of a personal fumble while not in CC.
So the answer is, yes it sucks to be hit with Must Parry. Stun Relief will not counter it.
Unless it's a "Stun Must Parry" effect. Then relieve one relieves all.
But it could be much worse.