I don't visit this forum as regularly as I should, so this might be a little late... anyway:
Some time ago, I've played around with a set of 3 TP-Link AD7200 routers , which have 802.11ac wave 2 radios with MU-MIMO capabilities.
I had the 3 routers flashed with OpenWRT LEDE (v17.01).
One of them was configured as an AP, the another two were configured as clients.
I could see the AP advertising MU-MIMO beamformer capabilities (as evidenced by the 'VHT capabilities info' field in Beacon frames, e.g. here).
The clients also showed MU-MIMO beamformee capabilities (e.g., here).
However, my packet captures did not show any MU-MIMO sounding frames or frames containing feedback matrices, which suggests MU-MIMO wasn't being triggered.
I then tried the same experiment, only with the AP flashed with TP-link's stock firmware, leaving the clients running OpenWRT.
In that case, I did see MU-MIMO sounding frames and feedback frames being exchanged back and forth!
I could even see the bandwidth being shared between the 2 clients after MU-MIMO kicked in.
My 'study' was by no means exhaustive - I've only tested one type of hardware - but maybe this suggests that OpenWRT has some issues with MU-MIMO when running in an AP.
I have more details here.
I hope this helps someone!