Hi Arnaud, I will not enter the debate pro/con Loxone here because it's off-topic, but personally I don't want to use Loxone Tree because I want to use hardware which is not limited to one brand (KNX is a multi-brands standard, Loxone is a proprietary brand and protocol), and also avoid that the whole electrical installation relies on a central server.
But the Loxone Miniserver is a capable and relatively cheap superviser and logical module for a KNX installation, which can also be used as an IP gateway. It would be even bettter if it could allow 2-3 simultaneous tunneling connections though :-)