Ticket T195503 describes the problem with Category search and suggests some ways to mitigate. Whatever we do, we'll need to write Help page that is very instructive about categories. This should include, at a minimum:

1. A description of the category system and its shortcomings.
2. Strategies for getting around these limitations, including the use of Saved filters with multiple categories (which seems like the only real way to use this feature).
3. Possibly a description of the Wikiprojects and suggestions for using them as categories that actually include the subjects one thinks they do.