In SOQL, if you don't specify the returned fields and then try to query them, you get an error. Is it the same in SOSL?
List<List<sObject>> values = [FIND 'acme' IN ALL FIELDS RETURNING Account, Opportunity(id, name)];
List <Account> a = values.get(0);
String name = a.get(0).name; // will this line produce an error since we did not specify the name field in the SOSL query