Re DC operation of appliances with universal motors, while the motors will be ok with it, the switches in modern appliances generally won't be. The gap between the contacts when the swtich is opened is likely to be insufficent to quench the arc.
Anyway it's more hypothetical than anything as DC mains no longer exist. There was however a commercially made gadget a few years ago which provided 240V DC from your car alternator. It was recommended for various heating appliances, power tools and incandescent lighting.
As the output from a car alternator is unfiltered rectified DC, any switch arc will be extinguished as per AC during the zero voltage part of the cycle.
I have never seen 50 vs 60 cycle operation of induction motors being a problem except for speed critical applications.
