Hi Joaquin,

TLDR: Wiki Replicas' architecture is being redesigned for stability and performance. Cross database JOINs will not be available and a host connection will only allow querying its associated DB. See [1] for more details.

If you only think of Wikipedia, not a lot will break probably, but if you take into account Commons and Wikidata a lot will break. A quick grep in my folder with Commons queries returns 123 lines with cross database joins. So yes, stuff will break and tools will be abandoned. This follows the practice that seems to have become standard for the WMF these days: Decisions are made with a small group within the WMF without any community involved. Only after the decision has been made, it's announced.

Unhappy and disappointed,

Maarten