Disadvantage of electrical resonance

The noise could interfere with other sensitive electronics

wat does resonance has to do with this....noise will anyways interfere with other sensitive elements...

Unwanted resonance could cause oscillations or instability and make a broadband amplifier response to be non-flat.

Keith.

It causes unwanted impedance variations (even if the resonance is far off application frequency)

"resonance in electrical circuits?". As this is posted under RF, I suppose you mean "resonance in RF circuits?", at resonance components don't act like their label says, a coupling capacitor with long leads can go from capacitive reactance (wanted) to a short circuit (could be wanted) to an inductive which is unwanted.
Frank