That hasn’t been my experience at all…
So maybe support is limited to certain hardware.
Ex.
Haswell laptop, nvidia 755m (optimus), a wayland session falls back to Intel
Coffee lake laptop, nvidia 1060 only (no Intel iGPU), falls back to llvmpipe
In the later case it maybe should fall back to nouveau?
Though on that hardware nouveau is only possible in very limited circumstances (- https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1822482