Sorry for delay, was on holiday...
Thanks for your feedback! The problem with the search in WordPress is that for custom post types it is quite difficult to do all the stuff you mentioned. Currently it searches for the "download" post type. Since these taxonomies are bound to that it should work on paper, but I guess WordPress search just doesn't catch it up (like it should).
Currently it's not possible to add taxonomies to search queries directly in the search form -- only the post type could be given as hidden parameter.
Other solutions would be to switch to plugins like Relevanssi but then it is mixed up again with results from default WordPress search.
I try to have a look to better take into account these taxonomies. If I find a solution it will be implemented in next update for sure!
Thanks, Dave :)