I tried flaked barley in an APA and will never use it again in a pale beer. I get a raw, grassy flavor from it that I just didn't care for.
I experimented extensively with flaked barley in pale beers and can confirm that it does produce an undesirable raw flavor that does not meld with pale beers. I think it works in roasty beers because the roast flavor overwhelms everything else from the raw barley.
I know that Lewis mentions in his book on Stout that oats in a stout produced a poor, astringent flavor. I don't understand why he found that, but I guess its possible. To me, it seems that oats are just another cereal grain and should produce a similar result. But maybe there is some other compound in oats that tastes worse than other grains?
When it comes to body building in beers, I've settled on using flaked wheat. I find that its flavor is fairly complimentary and only a percent or two adds a huge amount of head building capability in my beers.
By the way, raw barley has 10 times more beta-glucan than raw wheat, so I guess that using a much lower quantity of flaked barley might work effectively without flavor effects. I just never got that low.