Technically, the critical hit descrition should say "a critical hit raises the attack's save difficulty by +5"
The way I handle it is, any attack that requires an attack rolls and allows a saving throw may benefit from a critical hit, gaining a +5 save DC. It's a slight advantage of having to make that attack roll.
If you want a somewhat more detailed and balanced version, you can say a critical hit raises an attack's save DC by +5 or an effective 10 power points, whichever is less, so an attack that costs 4-5 points per rank only gets a +2 DC bonus rather than the full 5. This makes critical hits less devastating for this big attacks.