I would think an accordion player would say “You are only playing half the instrument. I had to learn all these buttons over here under my left hand.”
Maybe that’s true, but people (and many melodica players I’ve seen on yT, not people on this forum) tend to forget that the melodica is a WIND instrument too. Breath control and tongue articulation really make a difference in tone quality and interpretation, but many keyboard players who undertake the melodica seem to not know that. And even the marketing strategy of Suzuki/Hammond maybe is accomplice in keeping alive this misunderstanding.
The melodica offers TWO types of articulation technique: keys and tongue. From a technical point of view that means we have to master both, but the instrument can be uniquely expressive, in a way no harmonica or accordion could be.
There are tons of materials on articulation on the baroque recorder (and oboe, many more than with modern flute) and tons of materials on breath control for any wind instrument: I think we should integrate those into our “regular”studies…