All the above is correct. The problem is that lightning also can happen outside of a thunderstorm cloud or propagate from inside to the outside.
The results of a lightning strike can be manifold, from nothing happening to holes in the structure to instrument and other components failure. The complete instrument failure can happen, but it is rare, and according to my observation rather happened on the last generation electro-mechanical generation than on the latest gadgedery.
I had one a multiple lightning strike on a turboprop and after the flight it showed that the lightning entered the cell, exited and reentered again, thus producing three holes in the upper left front fuselage. Those holes can be from tiny to one or two centimeters in diameter (that's up to an inch). But surprisingly the cabin pressure isn't really an issue, because the bleed system can provide more pressure than air is leaking.
Interesting lightning strike had been observed when lightning entered the cockpit and/or cabin and generated some light shows.
Dani