In the western world, tremolo and octave models are rarely played by “serious” harmonica afficionados (the main exception being in the field of Celtic music). They are great for playing simple folk tunes and don’t necessarily require a command of single notes to create a pleasant sound. Here that is what they are mainly used for and this doesn’t really require a high level of skill. However, in Asia the picture is very different and there are numerous players in the Far East who play with exceptional virtuosity on these models, both as soloists and in ensembles up to full orchestras.
Regarding the tonal layout on such instruments, this is generally dictated by the fact that the interval between blow and draw note becomes greater the higher you go. After a couple of octaves you reach a point where it just becomes too difficult to play scale notes in sequence because you have to jump back and forth too far.
That said, the actual playing techniques needed to master the diatonic harp are more complex and in themselves more difficult to learn due to the fact that notes are created in a variety of different ways (blow, draw, bend, overbend), thanks to the unique qualities of the dual reed system found in these instruments. In the past, conventional wisdom claimed that the diatonic harp was a beginners instrument and the serious player was expected to “graduate” to the chromatic. Now it’s generally recognized that they’re completely different and each offers musical possibilities which the other does not. Basically however your analysis is dead right 