Stuns are a pretty basic mechanic of RPGs. Almost all games employ them in some form. They're fine, as long as they're "fair" (in the sense that it should be very rare or impossible to lock a battler into a chain of several consecutive turns stunned).
However, I tend to prefer "soft disables" such as silences, enrages, disarms, etc., which allow the player to make interesting decisions in adverse situations, rather than "hard disables" such as stuns and petrifies which take all of the control out of the player's hands and force them to simply watch as things happen.
I like the stun-break mechanic, although it can get repetitive when every single fight utilizes the "stun enemy, then kill them before they recover" routine, and it is kind of difficult to replicate in a turn-based system.
I do the reverse of FFXIII - you can't knockdown enemies but they CAN knock you down after you take too many hits (or simply with specific special attacks), in which case you will be vulnerable to attacks and unable to fight back until you stand back up and recover your weapon (which is player controlled unlike a typical stun that removes your control), this creates tension in fights where you have to defend at the right time to recover your posture and avoid getting knockdown to prevent a disadvantageous situation.
Normally I'm not a fan of giving enemies (in general) a mechanic that can work against the player, while not giving the player the same mechanic to use against enemies - it feels "cheap" and unfair, like the game is intentionally working against you. However, this one has me a bit intrigued. What is the purpose of this mechanic - is it to present a drawback to spamming heal moves (you can reverse the damage, but you can't reverse the Stun meter, which will eventually allow the enemies to deal two turns of damage while you're stunned and can't heal)? Or is there some other purpose you had in mind when you designed this Stun mechanic?
If I'm right about the purpose, I might suggest reworking the mechanic to be based on total damage taken (perhaps every 150% of a character's Max HP taken will stun them for 2 turns), and also allowing players to use this mechanic on enemies (which will almost never happen anyway - only against enemies that can heal or have a healer in their troop, since otherwise you'll kill them long before you deal 150% of their Max HP), which will feel more "fair" and allow you to create enemies with powerful heals (the key for the player will be to activate the Stuns at the right time and then unload on the enemy while they can't heal themselves).