IOTA's primary pitch is to be infinitely scalable and free. This requires that users run nodes that validate transactions, and that these "users" can be things like a toaster to be applicable for IOT. Tangles use tiny amounts of PoW to validate and prevent spam. But...if the PoW required is so low that your toaster can do it, than it's also low enough that a bad actor can mess up the graph with cheap spam.
2018-02-19 | Full Review