@martim I'm running OpenHAB with a few MYS sensors (and other subsystems) and I really like the system system. It is complex with a steep learning curve but incredibly flexible and powerful.
As for your problems with MYS/MQTT I have no solution but to use the direct connection over ethernet instead. I have tried switching to MQTT twice and failed miserably both times.
The MYS addon requires the new MQTT binding which easily collides with the old one. (The new MQTT binding is almost ready for release so it may have been fixed, if you are lucky.)
I don't know if the cause of this problem is with the OpenHAB MQTT bindings or with the MYS binding but it was not trivial to fix with the instructions available. As I could not really see any upside of using MQTT except to have an "all MQTT System" I went back to the ethernet connection which works fine.
I would recommend connecting it as ethernet connection first and get it working that way. Then you can switch to MQTT if you want to. The biggets hurdles are installation of supportive packages (mqtt and serial transports) and getting MQTT to work in parallel with MQTT binding version 1. If you post your logs (WARN level for the Mys binding) I can try help you decode it.