Yes, that is a good point about decals (and Japanned or shellac finishes) on older machines.
None of my machines have any of those things, so not a problem for me.
I have wiped it over all external surfaces on my machines to clean them, and it didn't damage the paint or plastic parts.
The trick seems to be time. On your stuck machine, I'd advise liberally lubing the moving parts with ATF and then leaving it to soak for a day or two.
Might take a couple of tries, but I bet it would work.
Adding acetone to ATF makes the best penetrating lube I have ever used, but the acetone might not be a good idea on a sewing machine because of the risk of damaging the finish.