Indole butyric acid (IBA) and naphthalene acetic acid (NAA) both initiate the growth of roots and stimulate the growth of existing roots, but to varying degrees. K-L-N has both, while SuperThrive contains only the synthetic hormone that is primarily initator. Both also contain vitamin B1, which is also a root-growth stimulant. SuperThrive has other vitamins and minerals as well.
I find the greatest use of either (K-L-N is cheaper to use and has more hormones) is as a meida presoak or as an additive (one teaspoon per gallon) to soaking solutions for just-received, desiccated plants.
Continued, heavy applications will cause deformed flowers (I've personally only seen it in phals), but it will go away if you resume sane chemistry in your nutrient solution.
I stopped using it continuously at a drop per gallon when I graduated to metering pumps. The hormones are very unstable, and break down rapidly upon exposure to heat and light. Putting them in my fertilizer concentrate tank probably only let them be effective if i watered immediately. later waterings were probably ineffective.