What are the Pros and Cons about being sotted in an "standard excess " slot in Army National Gaurd, specifically pertaining to NCOER?

The actual job your performing goes on your NCOER, not your MTOE slot. In this instance, however, your "slot" isn't really a position, it's more of a function/appointed duty. If you're the GCSS NCOIC, then that can be your duty position, but it's really more of an appointed duty.
Your options are to either use "GCSS NCOIC", " Automated Logistic Sergeant" or simply "Automated Logistics Specialist Supervisor" but excess is certainly not a duty position.

This recently happened to me for a deployment. As a paralegal we don't work at the company level, in order to deploy with a company level element I had to be attached as excess in a different MOS slot. In this case I was an unqualified radio equipment repairer.

This had the pro of allowing me to be attached despite my MOS, the con showed up when I went to re-enlist overseas. As I was excess in a different MOS slot, they had difficulty re-enlisting me and getting me a bonus. This was eventually solved with days to spare when the packet went to our State CSM and TAG, and then to NGB.

So while I am unsure how this could affect your NCOER, I'm sure there are some unknowns that may throw a wrench into it. I'm also curious how this same situation will affect my upcoming Specialist Eval when I make SPC next month before transferring out of my unit in September, in which I am still attached as excess.

I hope it works out well for your SGT.

Yes, different service and it's been a while, but the board precepts amongst the Services are similar because SECDEF says so. I had a lot of Board experience, both OFF and ENL. Precepts use "shall not be disadvantaged" for many things. Obviously race, gender, marital status, etc. Goes further. Nontraditional billets, joint, etc. The real issue is what you're actually doing, how well you're doing it, and how you're moving the Command along. Taking care of people also weighs in. Given that, you need to be an advocate for yourself in making sure the NCOER author, reviewer, signer have the opportunity to paint an accurate (presumably good) picture. My frustration with these reports is the terrible writing skills that are found up and down the line. It's been a problem forever. A good indicator of your Command thinking highly of you is the old proverb "No good deed goes unpunished". You do good, the Command hands you more diverse challenges. That gets into the writeup and Board members can see that. Smart Skippers purposely divide the promotion groups into thirds. It's best described as the Must Promote, Do Not Harm, and What's Left. You need that final third to have the headroom to make the Must happen. BTW what kind of words do you find in these groups?
Must: Superlative, innovative (Why the hell is this SM not promoted yesterday?)
Do Not Harm: Effective, timely, complete (basically competent)
Left: Potential, pursuing, etc. (work in progress narrative with no completion timeframe)

Bottom line, charge forth every day and make the positive difference. That's your best chance to force the paper to be a Must.