Is it recommended performance and feature-wise to use OpenWRT on Ubiquiti's switches?

The CPU in such switches is far too slow to do any data plane operations, so performance is entirely due to the hardware switching. Replacing Ubiquiti's OS with OpenWRT just gets you a different management interface for configuring the hardware features that actually handle traffic. (Unless for some reason you desperately want to have a VPN endpoint that would be limited to a few Mb/s at best.)

Ubiquiti make great hardware, however a lot of people hate the software as it is buggy and inconsistent. This at least gives those people options to run something a bit more opoen and manageable under the hood.