I wouldnt say drop checkers are "wrong". They do what they're intended to do if setup right. The tough part is you cant compare my drop checker to yours and get any sort of accurate idea of how much co2 either of us might be injecting.The short answer: drop checkers are dangerous because they are more wrong then right and give you a false sense of security. They are mostly wrong for a multitude of reasons that have been covered before, with a list of issues that would not fit into a single post.
In practice, a) start very low and very slow b) watch your fish for signs of distress - they are your "canary in the mine" and are the only indicator that matters.
Good advise here.A drop checker is a very good thing to use when first starting to use pressurized CO2. Without it most beginners will let their fear of harming their fish keep them from ever using enough CO2 to be effective. With it you can be sure you are getting some CO2 dissolved in the water, and enough to be of some benefit to the plants. Fine tuning the CO2 - the bubble rate - can be done more than one way, but as the nice chart above shows, the best way is to use a bit more until the plants no longer show any benefit from using more, and the fish don't visibly suffer from it. This fine tuning takes many days of observing both the plants and the fish, between tiny increases in bubble rate.