Emerson is a unique figure in the field of keyboard players within the rock and pop realms. Unable to deviate from the classical musician mentality instilled in him by his musical education, in order to adapt to the verse/chorus/bridge/solo/outro song structure necessary to define himself as a rock musician, he wisely chose to rely on musicians trained more or less directly in that field.
As long as he did so with mediocre musicians (The Nice's Lee Jackson: a decent bassist, an ordinary composer, and a terrible singer), his career did not take off; when he moved on to the right people (in particular Greg Lake, an interesting bassist, very melodic, a notable composer albeit lazy and unprolific, a powerful and moving singer until excesses and neglect prematurely damaged his vocal cords), everything went great.
Emerson was therefore not an... autonomous progressive musician, in the sense that he needed a rock base, a pop melodist, colleagues around him to create the ordinary environment of the most popular music of the time, in order to then graft his knowledge, his virtuosity, and his experiments with early synthesizers onto it—which, to today's ears, sound almost naive but were disruptive at the time.
ELP's progressive music was a patchwork, a semi-osmosis not fully evolved between classical and pop music. Typically in their records, a piano sonata with a central fugue (90% Emerson, 10% the other two) would be followed by an acoustic guitar ballad (90% Lake, 10% the other two), a liturgical organ toccata upon which Lake would laboriously adapt an operatic vocal melody, followed by a barrelhouse scherzo on an out-of-tune upright piano, and so on. The suites were truly such, in the sense of being structured into real movements, totally different in genre, atmosphere, and complexity if they came from one or the other of the group's two composers.
Pianistically, I have always admired Emerson's left-hand work: secure, agile, extremely fast, independent, virtuous. It was on the piano, his authentic instrument, that this musician gave his best and put in the most passion. This does not change the fact that Keith plays the lion's share also in the history of the Hammond organ, particularly regarding the sound: his choice in drawbar settings was always excellent; the use of the instrument's wonderful percussion to charge rapid scales with presence and incisiveness gave certain passages of ELP's music a beastly strength and danger.
He worked himself to death because he couldn't stand the fact that arthritis had eaten his right hand, preventing him from performing decently. The videos of his last concerts are tear-jerkers: the ring and pinky fingers shriveled, chords and scales played with three fingers... and evidently he did not have, and never had, the spirit of a Django Reinhardt, to whom this problem occurred at eighteen.
The gimmicks staged by Emerson for much of his career do not count, and should not count, toward the musical judgment of him. Daggers thrust between the keys, the Hammond played backwards and perhaps even pulled onto himself, whistling remote controllers with built-in fireworks... all merchandise for the shallow-minded, and to attract an audience even among those who give a decidedly superficial role to their attention and musical sensitivity.
For everyone else, it is enough to listen to, or revisit for the umpteenth time, "Take a Pebble" from beginning to end. Emerson did me good; I took him in the right doses, without overdoing it because he recorded quite a few abominations (as many did). Thank you and rest in peace.