Buttercups aren't very good either, but they taste very bitter so generally the goats will avoid them because they do not taste nice. You can put lime on your field to get keep them under control. We have buttercups in our field (bad area for them) and no problems with that. What kind of rushes are they? I think most rushes are ok, I know someone who goats graze the rushes in an ajoining canal to their field to no ill effects.
We live in an area which has lots of marsh grass, and the goats love this. We no longer have marsh grass in our field, because the goats like it so much they have eaten it all out! Again, no ill effect.
Regarding the bracken, we had friend that kept cashmere goats on a hill farm which had quite a lot of bracken, and they had no ill effects, but then their was plenty of other grazing as well.
Beth