I've used API test kits extensively. I have tested them against Seachem tests many times and have found them all to be accurate to what they claim on the box. I've found that 99% of the innacuracy has to do with user error.
I use them on some very expensive reef tanks and trust their results because of the results I get.
1. Following the directions exactly.
2. Do not rush the drops from the bottle. Uniform drop size is paramount. Lightly squeeze the bottle and let the drop fall on its own.
3. Do not allow an air bubble to form inside the drop, this changes the volume of the drop.
4. Hold the bottles vertically. The holes in the tips of the bottles release a different size drop for each reagent. A drop from the side of the tip is a different size.
5. Time the tests as per the instructions. Certain chemical reactions must occur, especially on all calcium tests, before you can get an accurate reading. Seachem writes their instructions more thoroughly to slow you down on certain tests to help the end user achieve greater accuracy.
6. Keep your vials clean. Bleach most, use acid on calcium, magnesium, strontium, and carbonate test vials.
7. Use the correct sample size. A Variance of just 1mL on a 5mL sample yields an error of 20%.