The reason for crossing the pigtails on UBNT antennas was due to direction of the rf elements in respect to polarity of the propagated wave. This polarity does matter as you do not want your received wave to be out of phase with the receiving antenna. Most high quality antenna manufacturers follow the same standards which are reversed of WIFI which UBNT follows.
With mimo, the link is created with one radio chain and each end will determine the best radio antenna pair to receive the signal (notice not transmit). Basically the two most in phase with each other. The second chain is on the remaining radio antennas which may not be anywhere near the same phase and still make a low quality link.
Keep in mind that this is completely different from OFDM-MiMo which truly does not care and uses all radios and antennas at the same time on all chains.
Personally I would like to see the epmp line use 5mhz ofdm chains and we select the center frequency and number of chains ( width) with higher modulation schemes that are determined by the 5mhz single chain. More bps/mhz is always good.