Do flat fenders work fine?
I can't speak as a user of them, but I can as one who has followed them. They seem to work reasonably well, but they aren't as good at protecting people behind you - the arc behind them is narrower than the traditional curved in guard. I would think that in this regard, wider would seem to be better

In this they are better than the horizontally flat guards you see, which do nothing to protect those following you. 

If you're only worried about your back and legs, these seem fine - but if you're concerned for your fellow travelers, I'd be less keen on them.