I used Merlin on my TM-AC1900 APs for some time. It's more or less a slightly customized build of the AsusWrt firmware, with a bit easier time for fiddling, like adding packages and shell access. The webui is the Asus ui, with some tweaks. Merlin only runs on asus devices.

OpenWrt is distribution that works more or less the same on all of the supported hardware. IMHO, the Asus firmware is reasonable, so it's not a bad place to start, but some manufacturers have pretty minimalistic firmware and IMHO, something like OpenWRT that provides fairly consistent capabilities across a range of hardware is really handy (when OpenWRT supports your hardware anyway).

I run OpenWRT on an LTE gateway, and on my new APs, although I'm running off a fork right now because the ath11k drivers in 24.10 didn't work well for me in the rc builds; hopefully after this release settles down, snapshot builds will move to the newer linux kernel and I can switch to that.

I have used merlin for quite a while, mostly happy (except for some security holes...) However, once asus drops support for older devices (e.g. rt-ac68u and rt-ac86u), merlin might also drop it. For now rt-ac68u is dropped by merlin, but ac86u is fine for now (at least until the end of the year.)

Upshot: if you care about very long term support, openwrt is nice.