Small cermet pots should be no trouble at 50MHz, assuming you aren't
going to extreme resistance values. You will always have parasitic
inductance and capacitance, but if a carbon pot would work in your
application, a cermet of the same size and general design should behave
similarly.

Beware of the conductive plastic ones...some of those are "hybrid" pots
that have a wirewound element under the plastic, to get very good
linearity, and the conductive plastic provides interpolation between
the turns of wire. I once used one of those to read out an angle, and
it worked great, but I wouldn't consider trying to use it at RF. If
it's not a hybrid (no wirewound element under the plastic), it should
be OK.

You may also want to consider the amount of (RF?) current in the wiper.
It's generally a good idea to keep the wiper current low, unless the
pot is specifically designed for high wiper current.

Cheers,
Tom