The problem with some of the early pieces was the manner in which they were performed - and defective performances of music like this tend to tell against the piece itself to a greater extent than is usual. Glass' early recordings of Music in Fifths and Music in Similar motion are pretty duff in terms of evenness, precision and ensemble - perhaps hardly surprising given the circumstances of how he was trying to play them. Music in Fifths is a far more difficult piece to play anyway - presumably performances were abandoned early on for that reason alone. Glass' ensemble lacked instruments with a strong attack - the 1973 recording of Music in similar motion uses 3 electric organs, 2 soprano saxes + flute. Given the high volume and what seemed to be pretty poor amplification + mixing both in recorded and live performances, it's hardly surprising that the result was imperfect. However, the recorded performances (and, I gather, the live ones too) sped up so much as to make one suspect that some of the players, at least, were not up to the job.
I used to play these pieces (and others) with a friend some 30 odd years ago. Sometimes we used two electric organs, more often two pianos - the latter seemed to work surprisingly well. We were never amplified but tended to play in spaces which could easily be filled with sound. Our speeds were somewhat faster than Glass' and (amongst other things) precision, evenness and consistency of speed were important - because it was obvious when those things were even slightly defective. A pity he seemed to gradually abandon rhythmic interest over the next few years . . .