bassplaying.com where bass guitar takes center stage

I just ran into this chord, C7#9, while playing a BiaB blues jam. When improvising a new tune I usually start on the root and see where my ear leads me from there. With this one though the root sounded horrible. Some of the notes from C major sounded Ok but many didn't. I thought I might be out of tune for a few minutes but a quick check showed I was dead on. So I kept repeating the tune and tried some different things.

I found the the Eb major scale worked much better than C major, even on the downbeat. Now I'm not surprised at the Eb, after all it IS the #9 but I didn't expect the whole scale to sound good over the chord. If you have some looping software give it a try. So...

what's up with that? Why does the root and notes from C major sound so bad? And conversely, why does the Eb and notes from Eb major sound good?

Are there other chords where the root and simple scale/triad don't work?

We all tell new players to stick with roots and simple triads at first. So should we be saying 'with some exceptions'.? Or are these strange chords so rare we needn't bother?

Thanks all.