If we're talking about keys, then in most sessions, the melodeon is the most limited instrument there. A D/G melodeon player rarely needs to worry about playing in keys that others can't join in with.
There's got to be some give & take. Yes, there are some keys a D/G can't cope with like E & Bb and some that are doable but a bit limited like A & C which have
some missing notes and/or wrongly directional bases, (..and I didn't even mention D-bl00dy-minor!) but I have no qualms about re-keying a tune fo fit my box. Most fiddle players play Atholl Highlanders in A. I can play it in A but I much prefer to play it in D as I feel it fits better on the box. Incidentally, I also have a smart-arse version in Em just in case anyone complains.... only tried it once and cocked it up big-time... Why is it that you can play a tune all day long at home then as soon as you start it in a session it all goes sh*t-shaped...