“A 20% tax at any one stage in the industry is more likely to invite evasion than the equivalent of a more modest 6+% tax at three different stages. In addition, a succession of taxation stages offers convenient checkpoints to monitor the market.”
That’s an excerpt from written testimony of CANORML’s Dale Gieringer to California’s Blue Ribbon Commission on legalization.
The Center for New Revenue 
You then have to worry about compounding which will drive small businesses out of the business. A large integrated producer will pay only once.