Why does exporting make my video so dark? Never had this issue before.

First, especially if you have a full GPU card in the computer, uncheck max quality render ... it's only useful without a GPU in a very few circumstances, and can cause slowness and artifacts in exports otherwise.

Unless you are working with 10-bit media (and if you don't know, you ain't ... ) ... turn off the Max bit depth also. For the same reasons ... it can slow things down, doesn't help with 8-bit media, and not always with 10-bit, and can cause odd things itself.

Skip the 2-pass, you rarely if ever get any quality boost anymore but you do get a lot longer export time.

And I'm not sure on a 1080 that you really need clear to 32 on the max bitrate, 20 is probably entirely adequate.

Now, as to the differences you're seeing, they're really very small ... in relative terms. Why?

Because Safari is a color-stupid browser, YouTube is rather odd in the way it manages color (or mangles it), and ... no other screen on the planet will EVER EVER EVER see exactly what you see on your screen. Period.

Even in a highly color-managed environment with thousands of dollars per monitor and thousands spent on calibration in perfect viewing environments, you can't make any two screens totally match. It's the bane of colorists, with Client Attended Sessions. Client's are on the couch, watching the incredibly spendy 50 inch TV calibrated via LUT boxes ... colorist at workstation with a $5,000 monitor.

Client looks over at one point, says ... I like it on your screen better, make this one look like that.

Um ... no. Just ain't possible.

Realistically, for taking something out of PrPro which will do its best to make the monitors it displays show the media in Rec 709/sRGB gamma 2.2/4, exporting to YouTube and viewing outside any color management on a color-stupid browser, that's an amazingly close comparison between the two.

Be grateful.

Neil