For me the Roaming works quite reliable between 5ghz and 2.4 on the same machine. No idea if roaming between multiple routers works and how to set it up ...
Regarding the peak throughput: on current master I get max 35 MB/s from my synology with mt76, but >40 MB/s with the proprietary driver. On linux-4.14 the speeds were better with mt76 compared to the proprietary driver, but only slightly.
I think it’s fantastic what the mt76 people achieved, but I had constant trouble with disconnected devices, unreachable WiFi printers and sound devices (which still persists in the most recent mt76 version). The proprietary driver simply just works in my set up. As soon as the mt76 works reliable for my needs I will happily switch back.