In RMU, when stunned you take a penalty to all actions (-50 for Stun[-50] but potentially could be -25 or -75), need to make a mental focus maneuver to maintain concentration, and foes gain a +20 bonus to hit you, but aside from those things you are free to take any action you want. It does not prevent you from attacking or defending.
I like the change (yay !). I will probably use it next time, with a little tweak - the default stun effect is -25, but the modifier is cumulative (so if you're stunned for 3 rounds and, in the next round (where you are at -25), are stunned again for three more rounds, the modifier becomes -50 until the first stun effect expires. I don't know if the accumulation is a thing in RMU (might not be since stun modifier ranges from -25 to -75 with -50 being standard), but I like the idea.
As a side note, the change makes stun effects less powerful, and, as such, more usable in a "select your wound effect" crit resolution mechanism that was discussed earlier.