[unsung]

On a technical note, the first point is not really true. Using atmospheric models like WRF get you pretty good irradiance forecasts that are roughly proportional to PV output. I (very briefly) worked with a group doing just that for power companies in the southwest, and it was pretty decent out to 5 days or so. It eventually got picked up into another program [1] that would contract out to big PV installations.

[0] https://www2.mmm.ucar.edu/wrf/users/

[1] https://forecasting.energy.arizona.edu/sveri/