I think it's more than just flash. To me, Oscar's playing is just TOO "perfect" much of the time, and being "perfect" just didn't interest Miles (and if there's anyone in jazz who REALLY didn't give a shit about perfection in the conventional sense, it was Monk). When I listen to Oscar as a learning musician, my goal is to try and take away how smoothly he makes the transition from one chord to the next, how all the notes are just "right" for the phrase, and how he manages to finger everything so cleanly. If you want to learn the rules, you listen to Oscar. If you want to learn how to start breaking them, you listen to Monk. Working on both is essential to me.
That said, I do enjoy much of Oscar's playing, especially his trio arrangements. The emotional connection you get on a Sketches of Spain isn't there, but occasionally he can approach that level.