All the things Steve Shaw complains about can be changed.

The D/G melodeon is quite highly pitched, which means it is best played in the lower octave so lacking the low notes can be a problem. Instruments in other keys are often played in the upper octave, which avoids this, but the upper octave on a D/G can be very high and squeaky, especially on the G row.

Some melodeons have low notes instead of accidentals (although these are not 'useless' once you become more advanced). Many D/G players now favour 4th button starts (where the scale starts on the 4th button rather than the 3rd) which allows both low notes and accidentals.

Bellows technique is a question of practice, and you stop breathing in sympathy with the bellows after a while.