I'd have said no keyboard, till I heard both regular piano and keyboard one used with great effect (and tastefully not dominating) at oldtime jams. Used with fiddle tunes it can be a great help to chord-players to catch the chord changes. There are a lot of parlor, oldtime, blues, and jugband songs for which piano is appropriate.

So I think it dpends on how it is being used and how it affects what happens. The focus should be on the music, not the instrumentation used to create it.

So often, we worship "tradition" and insist on using only "authentic" instrumentation. But it's kinda shortsighted I think-- since the "tradition" of oldtime music in particular was to use whatever instrument happened to be at hand in order to create and enjoy something that is informal by its very nature. What we insist on replicating so rigidly often turns out to have been complete innovation in its time! So I think we need not to hold onto what we love so tightkly that we strangle the life out of it.

~S~