Cannot say Wayland works smoothly on my triple-monitor setup. One of the monitors sometimes stops working randomly, and wakes up to display Plymouth screen when I reboot the thing.
Dell Precision 7520 with an AMD GPU. The degree of flakiness is different depending on whether you're on Plasma or GNOME, but it's there nonetheless.
I mean, my data point of one isn't all that helpful if you're having issues, but triple monitors do indeed work on my end.
I switch between a station that has a 4k display next to a standard 1920x1080 display as well as my laptop display (3200x1800) and my home setup with the laptop and 2 4k displays.
I had an issue on the 4k displays when I attempted to run two displays and a usb hub on a single thunderbolt line, but that wasn't Wayland, that was me being dumb: The thunderbolt protocol only support 40Gb/s and each monitor uses 20Gb/s and the hub eats another 10Gb/s. If the hub got detected last, it dropped to usb 2, if one of the monitors came online last, it would drop to 30hz refresh rate. Frankly - I was a little floored when I realized that I was the one being dumb and the system was mostly still just making things work. Just for shits and giggles I booted up an x-session after to see what it does. The answer is lots of black screen.
I'm guessing that's an AMD GPU thing and not a Wayland thing. I run a triple-monitor setup with an AMD GPU on X.org and I have a similar problem: every once in a while when I boot my PC one of the monitors just doesn't come to life. Restarting X usually fixes it.
Dell Precision 7520 with an AMD GPU. The degree of flakiness is different depending on whether you're on Plasma or GNOME, but it's there nonetheless.