Seriously, the only caveats I've heard of are some voltage limitations
for metal films - but not espcially quantified - perhaps a few hundred
volts? Mikkel's .25W mention is a new one on me too, but logical given
the size.

Voltage rating of the paint that covers the body of the resistor? Well,
just be sure that
it si in the air and not touching something else. Internal arc over?
Could be an issue....