Hi Grzegorz,
Thanks for the feedback, the plugin is meant to be built around multiple search fields, with categories and sub-categories used as the main building blocks. Have you tried out the tags or custom fields feature? It should be possible to build very complex catalogues using those features. You can check them out on our demo site:
https://www.etoilewebdesign.com/ultimate-product-catalogue-demo/
If products were allowed in multiple categories, then there would be lots of issues with multiple listings for the same item within a catalogue. The category restriction was taken as a design decision, to avoid the multiple listings issue.
We’d be happy to provide you with examples of complex catalogue built using the plugin if you’d like some inspiration!