Is the long tail you're talking about more in 2D rendering itself, or in accommodating the great diversity of the humans using the UI, e.g. accessibility and internationalization?

Rendering does have a lot of interesting 2D-specific problems too that 3D engines don't optimize for (Raph Levien's blog has great stuff on this), but what the 3D engines lack is often more related to UI layout, input (e.g. sophisticated focus handling), advanced text (editing, layout, shaping).

Unity's probably been investing the most into a more complete 2D toolkit (which is their third or fourth generation of 2D UI toolkit bundled with the engine) lately. Among the FOSS game engine projects Bevy has made strong 2D suitable for UI an explicit goal. But that one true converged contender is still not even on the horizon yet, IMHO.