Flourish is really just the micro nutrients that are contained in CSM+B. They assume you are getting your macros,(N,P and K), from food your fish consume. It works OK but if your plants grow fairly quickly they will use up those levels pretty quick leaving an imbalance. Imbalance means the plants will no longer grow well but algae will. That is what the EI method prevents. It provides way more nourishment than the plants will consume so they grow fast and outcompete the algae.
A years worth of dry fertilizers, (N,P, K and CSM+B), costs less than one bottle of each of the Flourish line including the shipping and you would need to dose 8 - 12 times the amounts listed on the bottle to equal EI dosing, assuming you have co2. Without co2 you would still need 3 times what is listed on the bottle. It makes keeping plants far cheaper and they grow much better. I wish I would have started doing dry fertilization much sooner than I did, my plants are looking great and it is much simpler and cheaper.