NASA put this out about a year ago.

https://earthobservatory.nasa.gov/images/150887/midwest-farmers-using-cover-crops-take-small-yield-hit

This isn't to say covers don't work.  I like and use covers, esp. rye before soybeans, but for me they are still a work in progress.  Which cover ahead of which crop on which soil type under which environmental conditions?  Terminate early or late?  Plant green?  Crimp?  When and how much of captured nutrients are released to the cash crop?  Are volunteer covers a weed in subsequent crops?  They add lots of new variables to  the management equation that we all have to try to solve each year.  Does that mean covers are a waste?  No, they have their place and IMO we'll all be required to use them to some extent sometime in the future if we want to participate in federal programs, including crop insurance.