Same here. Wood pigeons shred them on our site if they are left unnetted.
What they do seem to leave alone is proper mustard greens. Not the frilly delicate types, but the big leaved ones like Red Giant or Green in Snow. If you want a follow on crop to make use of bare space I would recommend either of those. They also stand the winter well.
They are hotter than mizuna and the like. I blanch them in boiling water, then put cold water over them and squeeze them. A lot of the heat goes that way. You can then chop them and they are a good addition to a stir-fry at the end of the cooking to add colour and some spice, or added into curries, soups, pasta sauce, any sort of baked dish like gratin, lasagne, moussaka or mac and cheese …. just a bit eye watering cooked as a stand alone green.