So does openwrt replace the edgeos on Ubiquiti gear? Trying to understand the advantages of this over the os it comes with.

Free software with full control

I'd read about any caveats first. The EdgeRouter, for example, has Cavium hardware offload that requires a proprietary kernel module. It's much slower without that.

Full control means you can recompile the kernel to match the proprietary module version, or try to reverse it.

I should know ; I have a HW accelerated NIC too. Works wonder until the max speed of my optic fiber.

As ubiquiti is the apple of routers and they do have very good routers and software, what is the motivation for putting openwrt?

Although their hardware is good I find their software to be a bug filled mess. The EdgeOS 2 series in particular has been disappointing. Impactful bugs that have been extremely slow to address and loss of functionally on some devices which was promised to be resolved but still hasn't. I still can't get IPv6 to work on my Verizon FiOS connection because EdgeOS doesn't support the required options despite years of customers requesting the options being supported.

At the same time as this they have attempted to slip a call home function in unnoticed and stopped living up to their obligation to provide source code under the GPL.

Edit: I checked after posting this comment and Ubiquity is once again making the GPL source archive available. It had disappeared for quite a while and there was a bit of an outcry regarding it.

Run OpenWRT on some Edgerouter Xs in my house and for me it’s consistency of the interface/configuration between all of my network devices and also full control. Runs OpenWRT quite well.

I am considering doing the same. What kind of performance are you seeing running OpenWRT? I know with EdgeOS performance is lackluster without using the NAT offloading module.