Let me chime in here since this is what I do.
First off you are correct about being unregulated. Companies aren't regulated by the FDA, and if you want to put Ginseng, Caffeine and Powdered sugar in there and call it something, YOU CAN.
Now can you mislead? You could, and some companies have, but companies that dont want their butt in jeopardy will try to do the BEST they can. Some companies will have GMP in place, and some go through audits to make sure they are GMP.
You dont have to be, but it helps. Again this is if you MANUFACTURE your own stuff in house. Many companies send product off to have produced FOR them.
There are STRICT guidelines for labeling we have to follow and what must be included on the label. As for amounts, we dont have to go by guidelines. If I want to call a product Caffeine blitz and add 450mg per dose I could, but more than likely the FDA would be knocking on my door because they know its too much per dose, and i'm pushing the limits. Also too, any good supp company knows when to stop. Have you ever seen 1g of caffeine per pill? No, and you never will.
I can say that sure there are going to be errors when manufacturing, and if a scoop says it has what ever in it, will it be exact?
Capsules can be easier than a plain powder or liquid. You have to remember that this stuff gets produced very quick, and as hard as we try to get it perfect we cant always. There may be residue on the mixer, or caught in a nozzle, or residue that got caught at the bottom when it was being mixed.
There are frauds out there, and the thing you dont want is to claim 500mg per capsule for instance and then it comes out to be 100mg. That's WAY off. What if it was the opposite? What if you did Iron and you were double?
With your protein in case, yeah it could be off a few grams, and it probably even is when you scoop it. Unless you actually weigh it on a scale you have no idea exactly.
To be precise you'd have to purchase a PURE protein with no sweeteners, flavors or anything, then weigh it on a scale till it read say 25g, then add it to your liquid. You'd know it was 25g cause you weighed it.
Your protein though will be closer than other ingredients because its a larger amount. Talk about mg or mcg and that is extremely hard to be accurate sometimes. Your label said 5mcg but you had 4.5mcg You know how easy it is to lose .5mcg in production?
Something will always be off, but as long as it isnt HUGE its ok.
The industry is getting stricter, and will become stricter.
My suggestion though is to not worry about something like protein. So what if your error is 2-3g on occasion.
Is one scoop a LEVEL scoop, a heaping scoop. or what? And at that, what is a heaping scoop? I wouldn't be too picky.
Most companies are legit, and great companies because its what they strive for. You cant always weed out the bad apples, but you can make a choice WHAT and WHO you will purchase from.
What you SHOULD be more worried about, is if that next supplement you buy does any more than a Placebo. This is more of a problem then what is in something.
You may buy something that has NO research behind it, or the ingredient, and guess what? There doesn't need to be, as long as they dont CLAIM it does something.
Lets say I put dandelion in a product, and say... May increase strength, and performance. I can do that, and you can buy it, and I dont have to prove anything. Afterall I said it MAY increase strength.
Look back at MMUSA and the liquid creatine. People were buying loads of it, and it didnt do ANYTHING, and had more creatinine in it than creatine. It was a rip off, but they sold it.
Its people like your friend who spreads thing that aren't true. A small margin of error? Yeah, because of manufacturing. Lying? No, except a few bad apples.