IMO, custom firmware is a means to an end, rather than an activity undertaken just for the sake of it. That is, I run custom firmware when it gives me features I otherwise wouldn’t have had, or because the original firmware has issues.
For a great majority of home routers, OpenWRT and the like open up enormous possibilities, so I have no objection there. For a managed switch, however, the returns are diminishing: most of the time, the complexity of a network falls upon its gateway or firewall, rather than the switch. Yes, there could exist complex VLANs with priority flow control and GRE tunnels, but if a switch doesn’t support that, it’s usually because it can’t, due to lack of ASIC support or necessary performance, rather than firmware not implementing it.
Of course, things get wild in the enterprise switch space, where switches rise to the forefront of network design, with things like per-user VLANs and “lite L3” routing. But I’m ignoring those, since they’re hideously expensive and beyond the entirety of Ubiquity’s product line.
So I posit to you: what sort of feature would you want to see in your switch that’s not there today? Would that feature have to be on the switch, or could it still operate if it was on your router?