pH test kits require judging the color of a test tube vs a color chart. I find it extremely difficult to get closer than .2 in pH. But the ppm of CO2 in a water sample is proportional to 10 raised to the pH power, so a .2 error in pH is a 58% error in ppm ( what you think is 30 ppm is really 30 +/- 17 ppm)

I think pH tests are very accurate, providing you can judge the color correctly. I don't think the reagent has a short time limit before it "goes bad". Judging the color is a major problem though. I find that the easiest color to correctly judge is green. As the color goes towards blue it gets increasingly hard for me to tell one color from another.