Disadvantages to MQTT

• It operates over TCP: TCP was designed for devices that had more memory and processing power than many of the lightweight, power constrained IoT devices have available to them. TCP requires more handshaking to set up communication links before any messages can be exchanged. This increases wake-up and communication times, which affects the long-term battery consumption. TCP connected devices tend to keep sockets open for each other with a persistent session. This adds to power and memory requirements.
• Centralized broker can limit scale: The broker can affect scalability as there is additional overhead for each device connected to it. The network can only grow as large as the local broker hub can support it. This puts a limit on expansion for each hub and spoke group.
• Broker single point of failure: It can also be a single point of failure in the network. A common situation is a broker device that is plugged into a wall socket with several publishing devices that are battery powered. In the event of a power failure, the publishing devices would keep operating but the broker would be offline. The network would be useless until the power is resumed.
• Security: MQTT is unencrypted by default. This makes it natively unsecured and requires you to take additional steps and absorb some overhead to make sure TLS/SSL is implemented. If not, any communication over MQTT, including username and password, is open to hackers.