Search ubercart attributes

I don't know, I've never used Ubercart.
If they use entities, and attributes are properties on those entities (or entities themselves), you should be able to index them out-of-the-box (might need additional Entity API integration, though). Otherwise, someone would have to write special integration code for it, by providing a dedicated Search API datasource plugin for this data.

However, you should first try this with the latest versions of all related modules. Maybe it already works there.

In any case, though, I'd say this belongs into the Ubercart issue queue, as any changes would have to be made to that module (or in an additional module related to it).

This is called faceted search, and if you use Google you will find recipes for doing this with Ubercart attributes. It will be easier to do when #895236: Expose attributes to view is finished, so if this is important to you please contribute to that issue.