Do you mean the 18-18-21? I know people that use that and end up with very nice plants and good harvests.
You'll find different opinions for the best ratio's to use in flower. Some say 1-1-2, 1-2-3, 1-3-2, etc... I don't pay any attention to those specific ratios. The 18-18-21 has everything in it and the plant will take what it needs. Regardless of the ration it's more important not to overfeed. You can grow healthy plants from start to finish with just a basic 4-4-4, 10-10-10, or what you're using. Overfeeding is what will cause problems. Completely eliminating N and feeding only higher levels of P/K can cause nutrient antagonism. And a complete reduction of N is not good because flowers use N to develop.
There seems to the mindset with some growers that any N in flower is bad and mega levels of P/K is good. There are many posts of plants midway through flower with yellowing dying leaves because they started feeding 1500+ ppm's of nothing but a bunch of P/K.
As for people hating on Miracle-Gro, ignore them and let them hate. Maybe they're trying to justify spending $100 on nutrients and in many instances get a poorer harvest and yield than people using a $10 box of Miracle-Gro. Whatever their reasons most have never even used it they have only heard that it's the devil on cannabis forums. The same soluble salts that many cannabis specific nutrients use come from the same factories as the ones in Miracle-Gro.