I do remember some well-informed person saying that it was difficult to play a melodeon really badly or really well.

I think it's a relatively easy instrument to get "up and running" on. In my own experience, I was accompanying performers on the melodeon within weeks of getting one for the first time, punching above my weight, certainly, but I was able to contribute something useful, if very basic. I doubt that I would have been able to do that with many other instruments. I can't imagine that I would have been able to do it with a fiddle or with pipes, for example!

Unfortunately, time has not made me a real master of the instrument, so perhaps the proposition in my first sentence is true!