I think if chickens have other choices, they'll stay away from poisonous stuff. But if you dump an armful of buttercups into the run, there might be problems.
I don't know about rhubarb seed-heads, but I know a neighbor of ours had all his pigs die because somebody harvested a bunch of rhubarb and thought it would be nice to feed the leftover leaves to the pigs. So I guess that's the flip side, right? They don't ALWAYS know what's good for them!
In line with the "some parts are poisonous, some parts aren't'" idea, what's up with Brassica oleracea? Kale, cabbage, broccoli, brussels sprouts, cauliflower, collards, and kohlrabi are ALL THE SAME SPECIES! But I've seen on different lists that some are OK to give to chickens while others aren't. Obviously different plant parts have been selected for during breeding (leaves, reproductive parts, etc) but are some dangerous and others not? If anyone knows, I'd be curious to hear!