I too had the same concerns about the API test at first which is why I spent 100 dollars on a good nitrate test kit (only 50 reagent packets!). It's a colorimetric test that you evaluate using a transparent comparator that tints a reference vial of water that doesn't have the reagent powder in it. The kit is used by soil testing facilities and researchers doing quick field work assays as well as water treatment facilities.
Every time I thought "that API test can't be right" I would use my expensive kit and found that I get the same ppm NO3 within the resolution limit of the API kit.
Is the API kit perfect? No, it's very coarse grained and the colors are not good to the naked eye unless you have seen a reference solution in person.
I guess at the heart of it, spend the extra money on salifert if it really bothers you that much. I spent over 90 dollars more and now I stopped worrying about it. Or just make your own reference solutions and spend 0 dollars (which I wish I did before I bought such an expensive kit).
Has it so far proven accurate? So far, yes. For both SW and FW it has been within the same ball park as my Hach kit. And IMO that's good enough for my application.
Fluval Spec V
7x Microsynodontis sp (1)
1x Cranwelli Horned Frog