Disadvantages
The MAL will not support features of the underlying protocol beyond the "least common denominator" defined in the MAL. Messaging features (e.g. threading model, QoS, etc.) are limited to a simpler subset that represent the intersection of all of the underlying middleware options. However, feature of an underlying protocol may be selected through configuration.
An adapter layer between MAL and the underlying protocol, plus specifications for language bindings, are still required. Implementations must adhere to these specifications for interoperability. Thus MAL takes on the characteristics of becoming new middleware standard in itself.
The MAL adapters and the MAL language binding specifications must be maintained as the underlying middleware standards for the plug-ins evolve. However, the use of the MAL removes any direct dependence of the application on the protocol technologies and therefore it is possible to isolate any evolution to lower adapter layers.
MAL precludes the use of service contracts as the centerpiece defining a data-driven service architecture.
Read more about this topic: Message Abstraction Layer