Keith,
The MQTT implementation on the BC188 uses the open-source Elipse Paho MQTT library, so its limitations will be replicated on the units side. The unit also does not currently support data encryption. What other limitations are you referring to?
There is some documentation around the module at the following links, in particular the “net.mqtt." variables in http://bluecreation.com/wifi/docs/melody.html#configuration-parameters , as well as the "net_mqtt” commands on http://bluecreation.com/wifi/docs/melody.html#commands . And then there is the exposed Lua API, as documented on http://bluecreation.com/wifi/docs/lua/net.html#module- net.mqtt
Hopefully that will help/answer your questions.
Regards
Matt