XMMP is not private, metadata is exploitable. 

 Even if the data server is run by yourself?  

 The other servers on the network still generate metadata, XMPP was not designed with privacy in mind.

The metadata shows who you talked to and at what time.

On the other hand, setting up a SimpleX server is even easier and no metadata is generated. 

 XMPP is not a private messaging platform. It was specifically designed to be an open platform. Different components have been strapped on over time in an attempt to increase its privacy and security, but its foundation is simply not based on the notion of protecting user privacy. It certainly has its uses, but it shouldn't be considered an alternative to platforms that were built on the concept of privacy and anonymity.