The statement about fog not working correctly with extended values for physical units is wrong. Fog works perfectly fine, but by default it’s limited to the display range of 0 - 1. You’ll need to manually set the color’s value higher so that it’s within range of the scene’s luminance. Just think of it as an emissive source, which is measured in cd/m². Using the Pixel Inspector to sample an HDRI skybox’s horizon Scene Color(also in cd/m²) can get you 90% there for a fog color that is both in the necessary luminance range as well as relatively similar color to the skybox(give or take, depending on how complex the environment is in the HDRI).
I’m not going to say the Lightmass comment is wrong, but I personally haven’t had any Lightmass-specific issues since the physical units improvements of ~4.20(or .21?) and I’ve been baking almost exclusively.
Directional Light intensity is not entirely Lux, although it’s a common occurrence with renderers. Lux is a unit of illuminance, which is the measurement of light falling onto an area. Unless you can block out all ambient light from the sky, the Lux measurement you get from an illuminance meter is not the sun light intensity - it’s the combined intensity of sun and sky(plus any bounce light or occlusion from nearby surfaces). Use the meters in the Eye Adaptation viewmode to measure illuminance, but do it on a diffused, white plane(in most cases horizontally placed in the scene). It’ll provide EV scaled for 18% gray(so similar to a spot meter), in which you can set the manual exposure or auto exposure to cover. Or, if you want the nominal EV(as provided by log2(illuminance / 2.5)) I have the modified shader that includes both for 4.26, and just nominal EV for older versions here: CustomUE4/Shaders at master · bleleux/CustomUE4 · GitHub
Extended luminance is a must if you’re working with physical lighting units since it allows exposure to be expressed with physical units(EV) as well, otherwise the min/max values need to be scaled much higher, such as a max value of 10,000+. With EVs and auto exposure, you just need to set the min/max value accordingly to your scene. There’s charts that provide examples of where certain EVs are commonly used, but you can use the meter mentioned above to get your exact values. Manual metering is no different since it all converts to EV, but it limits you to a fixed exposure.