I wonder if they're saying that because it runs slow. How does it run with and without the converter?
Without the converter, assuming a correctly adjusted belt, it should run at about half the speed of a normal running 110v motor. (or 1/2 the speed of the motor with the converter) No damage will occur.
http://www.sandman-collectibles.com/...or-voltage.htm
I would strongly suspect that a motor that has been converted from 110 to 220 would have a label of some sort on it (I'm pretty sure it would have to be for sure to be converted from 220 to 110, due to the risk of fire), and I can't see a lot of reason for it to be converted, most people would replace the motor, as has been suggested here.
Most of the North American featherweights that I've seen have had Canadian motors on them.
I read somewhere that this was because of 2 things:
1. The different power in the UK, but in reality, that doesn't stop them from making a 110 motor, only testing it without specialized equipment.
and 2. The real reason I suspect: If they shipped an electric machine with no motor from the UK to North America, it wasn't a complete sewing machine, therefore it was a lower or even possibly no tariff on it. Sneaky.
