I recommend NPS about half the time as the key CX outcome metric (dealing with mainly SME companies), but there are about 8 other approaches/measurement systems I also recommend depending on the business model and organization CX maturity.  Having said that, I went back to this 2013 talk by Rob Markey and noted that starting around the 6:15 mark he goes beyond why NPS into a discussion of why many companies fail to get results from their CX spend.  Even if you are NPS agnostic – start there and listen to what he is saying from the perspective of answering, “How do I get greater return on CX spend…”