Police still being able to conduct no-knock SWAT raids on what may or may not be the right house to make a marijuana bust. Young people still getting convictions that prevent them from getting student loans or joining the military. Street dealers still financing criminal enterprises through obscene profits, and selling unknown green substances that could be anything from oregano to poison. Billions of tax dollars still spent to locate, arrest, process, prosecute, and incarcerate people for buying or using marijuana. People still being killed over marijuana, or marijuana profits.
If marijuana were legal to produce, sell, buy, and consume, and those things still occurred, that legalization would have been a failure from a practical perspective, in my opinion.
Well that was a complete dodge of the question if there ever was one.
I'll just mark you down as, "The only failure could be not getting free and easy weed."