It's unpredictability apparently, you never knew what set it was and had to lose a Poke to find. That was the main argument. Dragonite is probably superior in most sets bar mixed, Multi-scale is pretty good especially with SR being less relevant.
I'm 1-3, haven't touched it for almost 2 years. Still it's fun and a lot better then BW. Trying to find some of my old teams ;D