I have observed two consistent challenges with planting a straight brassica mix: My deer never let it grow enough to mature and you need a good amount of nitrogen for best results. To put it another way: Never plant just one plant, or type of plant. Always plant in mixes that include cereal grains, legumes, brassicas and "others", like buckwheat and chicory (no "k"). Some mixes work better than others in the deep south; some work better Up North. In any region, mixes, or small plots with various things planted and rotated, work better than homogeneous plantings like we see from farmers. Mixes keep something growing year 'round, which means they are feeding deer during their stress periods.
Most of the folks who get excited about a single plant in their food plots do so because they feel it has helped them harvest a deer. While I am not opposed to harvesting deer, I can honestly say I have moved past that as the primary objective. When I see twin fawns and they are HUGE...that's when I know the habitat work I'm doing is on track. The deer I shoot are of secondary importance, really. YMMV