Brassicas are a natural and plentiful source of vitamin K. Clover (specially Red) is not a brassica, but is ,also rich in K. Vitamin K is necessary for all kinds of metabolic processes, It also happens to be a blood thinner. In humans if you are are already using a blood thinner you should be cautious about eating lots of brassicas. I give my rabbits cabbage once in a while, usually when I haven't finished a head in a timely fashion. I am too stingy with broccoli, but I have given that to them on occasion. Wintercress is a favorite of mine for the table, but the rabbits also get it. Over winter I will frequently get kale or collards for the rabbits since they can often be had cheaply. I've given thought to growing chard for late year forage, but have never done it. I do grow turnips (cousin to rutabaga) for that reason. Well, the brassica family is huge so the list could go on a lot longer.
I think that the problem with bloat and diarrhea may be due more to gut flora than the species. Brassicas are an excellent media for growing lacto bacters. They are also good sources of dietary fiber. Add them together and to a gut that isn't yet infected and balance with the needed flora and that spells distress for any creature. One of the reason Sunchokes (not brassica) are often called farti-chokes is the phenols in them that are very conducive to lacto bacters. Eat them without pickling or cooking and your gut will bloom and that can result in painful gas. So it is with brassicas.
Now, I give my rabbits a broad range of green forage, so their gut flora is very diverse. Perhaps those rabbits that have succumbed to bloat and such after eating brassicas don't have such a diverse and balanced gut flora.
