I know that "angry adoptees" is a pretty common short-hand description for adult adoptees who some adoptive parents see as anti-adoption. I avoid using the expression because I think it can be seen as very dismissive of the concerns of adult adoptees. It suggests that the anger comes from some unbalanced, abnormal mental state, and so can be disregarded.
Many adult adoptees have reason to be angry, and it is unrelated to how good or bad their adoptive parents parented. It has to do with loss of control, loss of identity, loss of culture, loss of heritage, loss of language, loss of first families, loss, loss, loss. And you can gain, gain, gain -- a permanent family, a different culture, a different language, a different heritage, more material goods than you can shake a stick at! -- and still feel loss. And it is perfectly OK to feel that loss. And it is perfectly OK not to. Not being angry is not an unbalanced, abnormal mental state, either! Adoptees who are NOT "angry" aren't wrong, and adoptees who ARE angry aren't wrong, either. It shouldn't be a contest!
I also think it's kind of a finger-pointing phrase, and not just at the adoptee, but behind them to their adoptive parents -- you're angry because your adoptive parents weren't good (or good enough) parents. As adoptive parents, we like to think we can innoculate our kids from being "angry adoptees" by being good-enough parents, unlike those bad adoptive parents who produced "angry adoptees." (It's kind of like how women on juries can be really hard on rape victims -- she shouldn't have accepted that ride, she shouldn't have worn that blouse, she shouldn't have danced that way -- as a way to protect themselves from fear; I don't do those things, so rape can't happen to me (there are probably better analogies, but that's the one that comes readily to my crim-prof mind!)).
I wish it were that easy to control what our children think in the future (I can't even control what they think now!)! I admit it, I do all this reading and writing to try to "do it right" as an adoptive parent. I think we ALL do! But I don't have any illusion that in doing so I can make it all better for my kids. There's no vaccine against grief, loss and anger.
I just hope I can give them the space to think what they want to think, and to express their thoughts and feelings without fear of disappointing me or angering me. And when they are angry, I hope to be able to understand why they are angry without getting defensive or feel that I need to justify anything to them. And if they are not angry, I'm OK with that, too, so long as I can see that they really have dealt with issues rather than burying them.
I'm not making this as a blame-laying statement, or to suggest that others are wrong to use the phrase, just an explanation for why I don't. I accept that most who use the phrase aren't intending to be dismissive or blaming, but just to be descriptive. I just think the phrase comes with too much baggage to use as a convenience.