To answer the OP's original question, any instrument played insensitively can ruin a session. It's not the instrument, it's the player. The melodeon can be very loud (although expensive boxes can usually be played more quietly than cheap ones because they have better reeds), but so can other instruments, and some which are relatively low in volume can be very penetrating. Guitarists who use inappropriate chord progressions or who don't understand the rhythm are my particular bugbear. And don't get me started on percussionists ... :)

To successfully join in a session means blending rather than dominating, understanding the genre being played, and listening to the players around you. You can do that on any instrument, including the melodeon. And it is true that many musicians fail to do these, including melodeon players. But it's not the fault of the instrument.