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.
That's the thing to watch. I assume that those Porter Cable tools specified for DC operation are fitted with the appropriate switches.
Anyway it's more hypothetical than anything as DC mains no longer exist.
I think India is still supposed to have DC in some areas.
While it is unlikely that an occasionally-used motor will burn out if connected to the wrong frequency, one which runs continuously will not last long. I think this applies to induction motors only. Universal motors are safe.
That's pretty much the way I would take it. Lightly used power tools etc. run on 50Hz aren't going to be a problem. I'd be wary of trying to run something like a refrigerator or air-con compressor on the wrong frequency.
