The problem was to be solved with GEDCOM and its "data dictionary". But most programmers in genealogy have trouble understanding the GEDCOM standard, or the program they have written is not robust enough to use all of the data constructs and therefore the import/export does not work correctly with data from programs that do use it more. The GEDCOM standard also was not enhanced to support some of the newer data concepts that have been devised.
Sorry that this got off topic.
If you plan on using both online and desktop based genealogy software, be aware that YOU WILL loose data. I know of only one case where you can not loose data and that is where you you the EXACT SAME PROGRAM both online and on your desktop.