My personal favorite is in a cartman voice: "Go away, Flash. Nobody liked you."
THE main issue I saw was that Flash was thrust upon us way too fast. Partially because they had no idea they'd be going back to the well this many times with that universe. I can forgive that.
The other half is the perceived threat he presents to the fans who ship Twilight with others. Not to mention that there's a distinct issue with the fact his mlp is a newbie guard who we haven't seen since then even after we had a baby in the crystal empire xD
You read alot about luna or Celestia having consorts that are guards but nothing official. For good reason I'll concede. But the stigma of royalty and a common guard never turns out well, and usually involves alot of heartache
If they'd given time to introduce him and not have him come in and be a foil for a plot twist now and then we might be more receptive. But alas, he's just Flash now. :/