MM depends on receiving back from the phone information on the channel. That uplink information has practical limitations on many cases, including from interference. In this paper, Bjornson, Hoydis, and Sanguinetti take on the assumption that there is a theoretical limit of spectral efficiency. Beginning with Marzetta, potential problems have been recognized.
"The pilot resources are limited by the channel coherence time, the same pilots must be reused in multiple cells. This leads to pilot contamination. ... it appears that pilot contamination is a fundamental issue that manifests a finite SE limit, except in some impractical special cases."
In 30 page dense with mathematics, the authors claim, "We will show in this paper that this is basically a misunderstanding,