Problem is that supporters are forced to give up using moves to cover their weaknesses if the rest of your team faint. Plus Meganium is a saurapod, so it makes sense that it can't learn U-Turn and never will.
Meganium is not meant to be played as a sweeper. In fact, the only sweeping value Meganium has is surprise value. What Game Freak needs to do is give her some good support moves so she can properly fill the niche she's meant to fill. Anything from Spikes, to Wish, to [insert boosting move] + Baton Pass. Hell, U-Turn or a better ability (imagine a Meganium with Drought or Magic Bounce. That would be awesome. And yes, I know they wouldn't make sense on her, but that's why I asked you to use your imagination) would do wonders for her.