I think what goes wrong with many FWB arrangements is that the F part gets neglected. If you establish a truly satisfying friendship (which is, of course, a kind of commitment and involves some mutual expectations), then the caring that you feel for one another will allow the trust and tenderness required for truly satisfying sex.

If all is it is non-monogamy--sex without promises or commitment--then you're not sharing with a person, just trading strokes with a stranger's body parts. How can anyone relax under those conditions?

It's worth whatever time it takes to build the friendship first, in my opinion.