My old 4 year old AC68U got on my knees with traditional QoS, I was lucky I got out 90/60 and the OpenVPN server gave about 25/15 not to mention the tough web interface.
So I switched to an AC86U last week and am really happy, traditional QoS now gives 95/95 (linespeed), OpenVPN gives about 90/85 and the web interface is lightning fast.
See that it supports mu-mimo and that sambons mobile and flat support mu-mimo (according to the flags in the wireless log) but it is rarely both used at the same time so I have no idea of the mu-mimo performance.
Smart Connect on the other hand works really well, an SSID for both 2.4GHz and 5GHz bands. Saw yesterday when the co-worker came home that her mobile and plate first connected to the 2.4GHz band and then when she came in through the front door they roamed over to the 5GHz band after a few minutes.
If I sum it up, the AC86U has long reach and substantially higher performance than my old AC68U, then Smart Connect works really well at least for me. So full pot for AC86U.