Ezreal's E doesn't feel like an instant.

Nope.

It was done intentionally so it has a downside and it's not 100% better than flash

It is instant, once the cast time finishes, anyway. I like it that way because then I can blink out of Thresh hooks and Blitz pulls if I time it right. Antagonising displacement supports is just so fun!

It has a cast time, so unlike Flash, it requires about 0.5 seconds to actually happen. The instantaneous comes in when referring to how long it takes him to move after the spell cast. This all means that it takes just a little bit of timing to dodge danger.

Cast time blink vs instant dash.

Pick your poison.