Yanmega variants with early trainer lock can ruin this. Fortunately you won't see too many of those. I playmaggie at league and have had success against everything exept yanmega/trainer lock. It's the only deck to beat me multiple times. Unfortunately there is no real counter to trainer lock other than to reversal up oddish/gloom before it becomes vileplume. It gets even more annoying when they rare candy T2 into vileplume. I've also tested against Zekrom donk and found that the early "disruption" of losing pokemon on T1 AND T2 can give it fits as it tries to recover. Also, the lower basic count means it can get donked a little easier. I'm sure my list isn't perfect, but these are the problems I've encountered so far with it. You are correct though, if it sets up; it's BEAST!
Best deck, probably. Unstoppable, definitely not.