RIght now, Search (from the user metadata search links) does not handle collections - they are not returned as results, but, more problematically, if a collection matches and would be a hit, the search dies with an error and matching datasets are not returned. Looks like this is an issue in SearchWithFilterHandler.java at least - that's where the attempt to cast all returns to DataSet is made.
A partial fix is to avoid letting the error mask datasets that match. A full solution makes collections valid objects to return, which may tie in with fixes for showing them as results in the tag page and other search mechanisms.