Hmm yeah indeed, flatbows are not that bad with something to boost the speed of your arrows. However, one can't say they're good either: even with Fav Winds, the pack of grasp rushing on you in the tombs will dodge your arrows. It won't happen with a longbow.

However, Divine Ambassador is right about the monsters AI. Monsters will not try to dodge your arrows, so if you're targetting monsters with a range attack (staying at the same place then) then a flatbow will be as good as a longbow, accuracy-wise. But a foe moving will de facto dodge your arrows, unless he's at a short distance.