Just got my Flint 2 yesterday and didn't unpack and start comparison tests yet.
I'm a bit on the edge whether even I should do that, because during the last couple of weeks the Flint 3 didn't give me any headaches after the initial set up and kinda "sits there and just works". My setup is pretty simple, I'm not using any QoS tweaks etc, and so far it was all right as a daily driver/main family router.
WiFi is still weaker than my ISP router, but after a couple of days of steady work the 6GHz band gives me full 1Gbps/400mbps (which is my ISPs down/up speeds) to my MacbookPro over 1 brick wall and about 5 meters distance. At first day it was barely 500/300, and I don't know what changed, there is not a lot of neighbours with 6GHz routers yet, so can't blame the radio congestion. Not using MLO as it didn't give any benefits comparing to using direct 5/6GHz bands, and was even slower than that. 5GHz and 2.4GHz are still a bit frustrating but with my apartment layout nothing to be a deal-breaker.
What I've noticed is that Wi-Fi settings seem not to be working when configured via LuCI. So what I ended up doing for my additional hidden networks is just to create them via LuCI, then they appear on the GL UI, and I set up the rest of the parameters there, like channel number, channel width etc. Not perfect, but does the job.
Added an old spinning disk hard drive to the usb port, installed transmission (not a very smooth process with all the libevent tweaks necessary, but works), set up DLNA - just works.
Set up 2 VLANs on one eth port to connect my RPi with HomeAssistant on it - also works with some config on HomeAssistant side as well - this way with one eth cable my RPi is both connected to a "no-internet" network with all the iot devices for controlling them and to normal lan network where it can get updates and where I can log into it. I've set it up just to be a POC thing to try whether it will even work, but this config stayed to be.
What was curious, is that LuCI has ethernet ports on "switch1" numbered backwards, i. e. "Lan 4" in LuCI is the "Lan 1" on the router and GL UI
Didn't have luck with WireGuard VPN server yet, I'm to dumb yet to wrap my head around client set up on macos side. OpenVPN server is nothing to write home about, it's there, it works, gives acceses to internal network etc.
I found it quite interesting to set up VPN or WAN things via GL UI, then go to LuCI and see how the port forwardings, routings and firewall rules got configured. Gives an impression that a lot of things are happening under the hood with GL UI, but also gives a power to fine-tune things with LuCI (except wi-fi, but I think it's because of the drivers not being open)
All in all, since my setup is simple, I didn't have to dance around the Flint3 for over a week after the setup (and with announced 4.8.0 coming soon), I am not even sure should I unpack the Flint 2 now or just keep the box sealed 
Looking forward to updates for Flint 3