The documentation for 4.18 and older was wrong. The “unitless” units do not behave like lumens, they behave like candelas. However, they are not candelas. Previously I thought they could be used 1:1 as candelas, however with the 4.19 implementation it appears they are much smaller than candelas. You’ll need about 1 candela for every 625 unitless units.
Directional and skylights are unchanged, but they’ve always been significantly brighter than point and spotlights, so they may compare well with candelas.
All in all, it’s great to see these are getting a proper implementation. Though, there are a few things I’m curious about:
IES profile lights, which really seem like the core reason to have physically correct lumens, but still seem to be standardized around unitless.
Source Radius and Source Length, as these have always been suspect, and are now extra suspect.