My brother supervises at a dispensary. They are not attractive robbery targets at all. They have a lot of security. Like a bank, they expect the amount of cash would bring trouble if they were not prepared. Unlike a bank, they don't dispense much cash (just petty change) so they don't even have to leave much in the drawers, which are emptied frequently and dropped into a safe nobody there can access.
They have cash-handling processes similar to a casino, but again, they have much less than a casino or bank to take.
Employee theft is a much bigger problem than robbery, because you can imagine who works at them, but even then, it's hard to get away with.
You'd be much better off robbing a busy gas station or the like.