Interesting, never noticed any difference between those two modes before, so I thought it was just a fallback when not using MLs. Will check tomorrow.
BTW they have “updated” the project to 5.7 today. If it were deprecated, they usually stop updating them. Anyway I think it’s an issue mainly related (but not only) to grooming.
It seems like MegaLights is somewhat cheating as the quality of shadows is significantly degraded. Screenspace shadows are almost completely gone with megaLights enabled.