The Big Problem With IOTA
While IOTA has accomplished many of the requirements to meet its goal, to be the backbone of the internet of things, it lacks one key technology it requires to reach that future; smart contracts. IOTA has free (except for a small amount of computing power) and rapid transactions plus groundbreaking technology, yet lacks the one last thing that an IOT system MUST, MUST have, smart contracts.
One use case of IOTA is to have IOT devices hiring their peers to do jobs, say if your Roomba hired a different robot to help it out, but this is impossible without smart contracts to bind the devices cryptographically. Without smart contracts, it is difficult for devices to truly communicate, and it is impossible to have binding contracts such as ones required to hire robots. Another use case of IOTA is to have a car act as a self-driving taxi when the owner is not using it, in order to provide a secondary income to the owner. But sadly, this is not easily done in IOTA either. Without any way to provide a binding contract for the car to drive someone somewhere, and difficulty in the car verifying your paying it, it is difficult to implement this in an elegant or simple manner, or even at all!
The reason why IOTA does not have smart contract support is because the Tangle does not support timestamps, and it is difficult to order transactions, which is definitely required to have a complete smart contract system. IOTA is working on a centralized implementation of smart contracts using an oracle, but the project is highly secretive and may take a long time to finish, and this oracle would be completely centralized, making IOTA a very centralized system, which is definitely not a good thing. In the future, it may be possible that there could be multiple oracle systems run by different groups, and maybe even a decentralized one, but at the current point in time, IOTA remains smart-contract-less and all smart contract solutions for IOTA in the future are looking to be centralized.