Re: maturity - I think it is the latter where the problem is very complex and the dev time reflects that, it takes a long time to implement all the features people need and are used to in X11 (or, used to when coming from windows/macOS). For instance, screensharing is still not widely solved across all wayland compositors. In Gnome or KDE Plasma, it works as you’d expect. But a smaller project like river (tiling window manager that I daily drive rn) does not handle screensharing out of the box and takes some extra configuration - and even then it’s got some oddities that I didnt have in KDE.
As for your issues with NVIDIA gpus under Wayland, yes I think it comes down to driver support from NVIDIA (or the open source drivers just need to mature some more)
When they first mention the open letter earlier in the article, that is where it is linked