Just out of curiosity what were your settings, lenses and how low was the light even. Typically some research should have saved you that trouble, but maybe that info can save the next guy.
If people want to learn about shooting log, picture profiles, ISO's, apertures, etc. with these cameras there's already tons of info on this site about that. Also tons on Youtube & Vimeo.
I don't feel like rehashing all that here.
Edit: If it helps, I was using slog 3 and s-gamut3.cine color. Averaged about 1.7 stops overexposed. The ISO varied a little from shot to shot. Don't remember what they were.
"I only used it for one night so I wish I could give it away or sell it."
Grrrrrrrr. No offense, but if you only used it for one night your opinion doesn't carry much weight. I've been shooting the FS5 for over a year, and it just keeps getting better. I shot the A7rii for 2 months, and I don't feel anywhere close to confident that I can adequately sing its praises or bemoan its shortcomings.
The disparity is a lot greater between having onboard variable ND and XLR's in a usable, professional camcorder body with all the requisite buttons and videocentric features, but what do I know. I just produce for a living, not a hobby.
Fs5 for sure. Auto Internal ND filter (shallow DOF in daylight without sacrifice shutter), and 240p, with the chance to get the amazing and easy to run XAVC-L (almost identical to the intraframe version XAVC-I), and clear SDI to an Atomos Shogun Inferno, that now can record 1GB/S ProRes RAW.
Slowmo, and shallow DOF, Its what your clients will pay, not the color science. If you have to make serious footage, go raw withe the inferno.