See ongoing discussion thread, but the tl;dr is that attempting to run both Outer Rim and Nexus Extended Endgame causes an error on load, regarding missing science packs. The current version of the error with Outer Rim v0.3.3 and Nexus Extended Endgame v1.0.0:

Failed to load mods: Technology antimatter-containment-fields-generator: there is no lab that will accept all of the science packs this technology requires.

Science packs: automation-science-pack, logistic-science-pack, military-science-pack, chemical-science-pack, production-science-pack, utility-science-pack, space-science-pack, cryogenic-science-pack, metallurgic-science-pack, agricultural-science-pack, electromagnetic-science-pack, promethium-882-science-pack, antimatter-science-pack

After doing some debugging from the Nexus Extended side, when it completes its data-final-fixes, the antimatter-containment-fields-generator requires Cryogenic Science Pack, but for some reason, that is not allowed in the Omega Lab (which may or may not be the only lab that accepts promethium science? Didn't test that far). When the Outer Rim mod is disabled, Nexus Extended's Omega Lab does have access to the Cryogenic Pack at the end of its data-final-fixes

I'm not sure which side the error lies on, but the Nexus dev tried to fix with the release v1.0.0, but that doesn't appear to have worked as of Factorio stable v2.0.69