It is not directly comparable because LwM2M was originally developed for device management, while MQTT is primarily used for transferring telemetry or other opaque data.

My personal gripe with LwM2M is that it uses CoAP (Constrained Application Protocol) as its primary transport protocol. Since CoAP is based on UDP, you’re forced to rely on DTLS for authentication and encryption. Managing sessions with DTLS can be quite frustrating, and unlike TCP, there’s no reliable indication of when a session is closed by your peer.

Another issue is that it doesn't strictly follow the client-initiated communication pattern and can receive unsolicited messages from the server. I have found this to be very unreliable in real-world conditions for Device Management use cases.