I've been reading through this conversation almost 2 years later.
There is one simple problem here apart from roaming: automatic rearrangement based on load balancing.
I have 4 Asus ZW XT8 on a wired backhaul. At first one could think that this is like having 4 random AP on WAP mode. But apart from seamless roaming, which more or less is the same as described in this post I find that the nodes automatically arrange the nodes not only based on the signal strenght but on the saturation of the node, like load balancers, probably because they support all 802.11 standards and some algorithms going inside these machines.
I've been digging more on this topic and there seem some centralized managers like the one provide by Ubiquiti with Unify also mentioned in the last post of @psherman and also from TP-Link something similar called Omada.
I don't think that OpenWRT or any WRT-like system can accomplish this because there is no Unified controller for devices, unless the routers are pretty well packed and use something like DAWN as suggested. Problem with DAWN is that this requires a ton of router capabilities (802.11 k, r and v) to be somewhat equivalent to a centralized controller like Unify or Omada.
I was thinking on building a very cheapo wired backhaul for a ton of devices connected all around the place without having to assigned each single device manually with a couple of budget routers (like those $20 on average, like TP-Link WR841N, but after some research, this is not going to be possible given that they lack of capabilities). So if one would have to aim for a bigger budget, and looking for this specific scenario, then Omada or Unify are the way to go (with an Unifiy controller or a Omada OC200 or something of the liking)
PS: I've watched the video from the Toast, and basically what he does is disabling all the options that make Unify great for for increasing a WAP compatibility. Basically the same as configuring each single node without Unify, manually, straight from CLI. No point on using Unify except for the comfort of being able to configure all nodes from a single interface. In the video, he removes Advanced options which are all those load balancing options that make Unify great to some extent.
Personally I see this from a home automated house, that may have like 100 devices with ease (for example I have just in light relay switches more than 50 units). This devices are constantly pinging the network, which saturate a lot of the nodes they sit in and I have to keep them on a individual WLAN (not only VLAN) because the nodes generally suffer a ton from them (this is why many people consider instead of moving through Wifi, going to other protocols like Zigbee to keep things away from users Wireless devices). So having some sort of load balancer here is pretty useful. Roaming and that is also useful if you are talking through Whatsapp or Telegram wifi connected but I find that load balancing is more clutch for wired backhaul mesh, something that WAP cannot handle by default.