I have the AX88U and AC68U. Tried them in AiMesh mode. The problem I had was when the clients were still attached to the farthest router/node, the clients would become unusable until it decided to switch to the nearest node/router, but this could be minutes. I tried it for a few hours and everyone in the house that moved around would complain of very bad wireless connections. What I discovered is that while my phone was connected to a far node/router, the latency would go up to very high levels, ping when through the roof, and what was a 110Mb/s connection to WAN became a 0.5Mb/s connection.
I now have the AC68U as an AP, set the power levels and the Roaming assist level, and I have a better roaming experience now. Seamless. Even at the point of switching, the connection is at least stable.
I will be swapping the AC68U for a AC88U to go in AP mode, with my AX88U as the main, so maybe this will make a difference in the experience.