Science is not about what we want, but how things actually are. With cover crop mixtures, the cumulative research results from 27 studies show that, at best, finding a mixture that will beat the best monoculture is quite difficult. https://acsess.onlinelibrary.wiley.com/doi/10.1002/agj2.20340

Out of 243 comparisons spanning 7 indicators (see below), only 2% of the mixtures surpassed monocultures.
In 88% of these comparisons, the best monoculture and best mixture performed comparably.

The notion that increased biodiversity is always better should be discarded in cover crops, soils, etc..

Contrary Science; Cover Crop Mixtures, Monocultures, and Mechanisms, http://csanr.wsu.edu/contrary-science-cover-crop-mixtures-monocultures-and-mechanisms/

Several people have asked which cover crop species were the "best monocultures" in this review. Here are the top 15 species (related species) for all the metrics.
Cereal rye and oat are the top performers.
Note that there were many more cool season than warm season studies.