Erik Larsson co-authored the textbook on Massive MIMO, which included an opinion that FDD MM would never be practical. MM requires the phone/UE to constantly report back metrics to the cell. TDD can do that efficiently; Larsson believes FDD cannot.
Verizon disagrees and is already deploying. Ericsson, Huawei, and ZTE have done successful testing with telcos. Not long ago, a senior engineer at a large telco told me the company is confident their FDD cells will work just fine.
Larssen and a team from Lund went out to the university parking lot. They used a 7 meter test rig with 128 elements. They tested at 2.6 GHz with a bandwidth of 50 MHz,