IMHO..Given that accordions are a different animal, I always gig with a guitar back up.
Two gigs recently, one where a support artist's guitar string busted in the middle of a set...he panicked and I could see why, he had no back up guitar. I offered him mine which he gratefully accepted. Lesson #1.
Lesson#2...a lead guitarist in the side project band I'm in came to a gig with only his Telecaster...he had problems with the Tele and he ended up using my Danelectro 56 U3 Reissue, which I had in the case tuned and ready to go. Annoying thing was the lead guitarist has a Strat. at home that's perfectly fine.

Both gigs had a floor full of dancers and a house full staring at you...to walk off with the excuse 'I don't have a working instrument' will get you no more gigs and a lot of egg on the face.

I know electronics , like FR-18's , would be very stable units, but I'd personally never do a gig without a back up. You'd only get caught out once and you'd never do it again. But that principle applies to acoustic and semi-acoustic accordions/instruments.

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