With a protein map, you very well understand where you should alter nitrogen application, i.e. areas where protein is above the critical value for your variety, reduce N and save money, areas where protein is below critical value, add N and increase yield.  Also with this map it can flag other potential deficiencies and help you target where in the field to do soil tests.  Yield alone does not tell you much.

Not sure what frustration factors you know of.  I think with any new technology, one needs to want to make it work otherwise, I'm sure anything can fail.  I had a guy once complain to me that my sensor didnt work... seems he removed the GPS from the combine after a few days and then wondered why a map couldnt be made.

And as for price, how much is your fertilizer bill and how much is your crop worth?  Many data that we've seen suggest that 20% could be made/saved.  If you're worried to start variable rate fertilization because you don't know where to make adjustments, a protein map will tell you much that you need to know... and if you're already doing variable rate, a protein map will validate your decisions and help fine tune your prescription.  This technology will come down in price but people have to start using it first.