The CRON is not neccessary, unless you have some sort of an import feature for posts/post types. If you add the products manually, then those should be added to the index as well during the save process.
If the cron job is neccessary, then are there any errors within the error log by any chance? I guess that something might be different within the cron. It is basically the same request, so everything is loaded as via the ajax indexing, but maybe there is something else running there paralelly. Maybe setting an execution limit via php.ini could help with that. If there is anything in the error log, let me know.