The pixels on the A7Sii are larger than other similarly sized sensors. That's why the megapixel count is lower. Larger pixel size means more photons absorbed in same amount of time. I've also had one connected directly to a telescope. It was an extremely fun night as I could use the Live Mode to see what the telescope was slewing to without taking a 15+ second exposure. It was pretty amazing. I want one for this specific purpose alone.
However, as someone that has shot a lot of video footage with no light other than a full moon using the A7Sii, I highly suggest getting the cable connected remotes for this camera. Trying to fly this body without a remote on a shoulder mount with lenses, follow focus, monitors, etc is brutal. The start/stop button is so tiny and hard to press, it makes you want to throw the entire rig as far as you can. The button is more along the lines of a reset button that you need a small pin to press.
It's pain points like this that make a stills camera that can shoot video much different than a true digital cinema camera
They are. I recently tried out an A7S, nice thing, but getting the A7S2 in about two months. 4K in camera recording is worth getting the Mk2 for.