I'm running OpenWRT for a few years now and it's absolutely great for simple setups (and even quite complex ones for homes). It is also possible to flash recent Ubiquiti hardware with OpenWRT; an AP WiFi 6 Lite flashed with it makes for a cheap (~100€) WiFi 6 access point with great range.
I have a similar setup (older Ubiquiti AP-AC Pros), and for me it was just a dislike for Ubiquiti's cloud-connected/distributed management architecture. I don't want my network hardware phoning home or depending on the cloud or even other components on the local network for any functionality. (It's sorta similar to how I feel about Apple; I really like their hardware, but am not a big fan of their software.)
I also just in general trust an open source project (sure, I know, I haven't personally audited the source or the build process) over close-source manufacturer firmware.
Basically I sat down one day and wanted to add another WiFi network to my access point.
Since the previous time that I had to manage the AP, I had switched my laptop from Archlinux to Voidlinux.
I tried starting ubiquiti's stupidly over the top java based management daemon on my laptop in order to manage the access point (which I used because I don't own a smartphone and even if I did, I wouldn't want to run any proprietary applications on it). To my surprise the software relied on mongodb which basically no distro packages anymore because of their crazy license change. I tried to spend some time getting mongodb locally packaged for my laptop (I refuse to install software without the package manager backing it up) but compiling mongodb is apparently nontrivial and I couldn't get it to work.
So I gave up on getting that working and just put OpenWRT on it because it provided me with a nice simple command line interface as well as an optional web interface. I was also able to quickly cut down the firmware to the bare minimum as I just wanted to bridge WiFi interfaces to VLANs.
I had a minimal, reliable and easy to manage access point which has been running 24/7 since that day for the last 218 days without a single hiccup. I no longer stress about having to run some insane daemon when I want to modify the configuration of my access point.
The sister comment is pretty much right; I don't have much trust in Ubiquitis software at the moment. Additionally, I know and like the interface of OpenWRT and I don't want to run a separate management server.
