I wonder if participants consider the “limitations” of the free reed as a sound generator, when considering the most appropriate melodica for the given music (or, most appropriate music for the melodica)? When the French harmonium was developing (C1860) there were many innovations (like percussion, where the reed was struck mechanically as well as blown) that were attempts to modify (improve) the sound and generally limited tone. I once owned a colossal American organ which essentially attempted the same thing through banks of octave and sub-octave reeds. The point is, though – you are never far away from the accordion! It’s gonna be the same deal, whether you have a harmonica, a melodica, an accordion or a harmonium: the sound-world will be that of the free reed and its particular harmonic fingerprint. Thus, just as the saxophone failed to end up as part of the modern orchestra, but found a home in jazz, so it shouldn’t be expected that the melodica is going to achieve usefulness as (for example) a classical voice: it won’t have the brilliance of a trumpet, or the woody richness of a clarinet, or the poetic quality of the flute. It does, though, have its own special, expressive sound world, and is a great “folk” instrument, again like the sax. The big advantage over the accordion, for the melodica, is the breath control and extension of the body – a real bonus for keyboard musicians looking for more expressivity that isn’t achieved electronically. Whew! (And I hope this is on the right thread.)
Further thought – yes, of course, that breathing contact is a part of the harmonica, too. But key thing about the melodica is the keyboard. Possible to play almost anything. Having said that, a harmonica genius like Toots Thielemans doesn’t seem too limited by his instrument!