Hello @erwin_k ,
It is not possible to run reports that consume information from multiple sites. If you are a member of more than one site, you will need to run the reports independently.
The disadvantage mentioned in the article refers to the fact that if you combine data from different installations in the same site, you will be able to run reports across all installations, which is not possible if those same installations are divided into several sites.
I hope I have helped.