The harmonium in India is a truncated version of the original harmonium of medieval Europe and troubadours. The pedals are lopped off and air pumped in by hand-operated bellows. The range, too, is considerably smaller. the Indian harmonium was designed to be played sitting on the floor and meant to accompany singers by supporting the singer's notes and filling in on the odd interval.
As an equal tempered instrument, it goes against the very grain of Raga music. It does so also on account of its inability to produce the essential glissandi and other ornaments of Indian art music. There have been musicians who have tried to be harmonium soloists, and some of them have even gained a measure of "stardom" amongst the ignorant laity, but it is suspected that they persevered on this ill-suited instrument because they could not reach the required competence with more suitable instruments. As an instrument of accompaniment, it is passable in art music, but no more.
It vies, often successfully, with the Sarengi as an accompanying instrument because vocalists and Sarengi players both produce Raga Svar (notes articulated so as to present the Raga itself) as a result of which the vocalist sometimes feels disturbed by the notes of a Sarengi player, unless the Sarengi player is so expert on his instrument and has such heightened musical sensitivity as an accompanist that he is able to sublimate his own artistry entirely and support that of the soloist. Since the harmonium produces only Thaata Svar (notes without character, being merely steady flat sounds), vocalists do not have this problem with it.
There is another model of the harmonium where the keyboard slides across the reeds so that it is possible to achieve transpositions without changing fingering. This model is particularly popular in Bengal where it is called "scale changing harmonium" as opposed to the "ordinary" harmonium. Shown in the picture is an "ordinary" model.