I have been using Firefox on Wayland for years now, and for the most part it works, well, perfectly well. It's very nice on computers with mixed DPI displays, I've had no issues with any of that. (I have run into a couple situations where dragging tabs has crashed, but it's too occasional to care. I just session restore'd and went back to work. Features like Picture-in-Picture work great and I even have a special setup for Picture-in-Picture mode so it will stick across workspaces in Sway.)

I imagine there's a lot of concerns that have to be carefully considered, as it is a very non-trivial change. Like, does WebGL work? Video acceleration? Will there be a performance regression for anything? How about accessibility, do screen readers still work as intended? (They should since for now that's all out-of-band, but you never know.) Does it have trouble with any specific vendor of GPU?

An interesting consequence of Wayland's design is that a lot of the information conveyed about X windows, like ICCCM properties, are basically non-existent in the Wayland world. This means that for a tiling window manager like SwayWM, it's actually a lot harder for say, pop-up windows to not accidentally get treated like other tiled windows. So if you use e.g. Dolphin file browser, the copy dialogs will try to "tile" since there's no way for Sway to know they're supposed to be popups. This was not an issue for i3wm. This is both fixable and possible to workaround, and in the grand scheme of things not my biggest priority (I'm much more concerned about issues like stability) but I think that issues like these are of prime concern for Wayland adoption.