Well, I have read and compared the experiences of several aquarists and for the majority it definitely does not work as an effective medicine against Cryptocarion, much less velvet, and almost the majority agreed to use it as a prophylactic since it seems to be more "mild" than the treatments with copper, formalin, etc. And in my experience I can tell you that it helped me with bacterial diseases, in the early stages.
To answer your questions, I assume that the effects of a medication are more effective throughout the days, and it is more stressful for the fish to undergo the dips IMO. I don't plan on using any meds, (I've only recently had the idea of paraguard as a prophylactic), I've done qt without meds for 90 days with success, although I'll be ready with Prazipro and Cupramine if needed.