every ea works just in a specific market condition, and if that condition changes it will loose. a good coded ea has security features build in to prevent loosing the hole balance if the change occurs. however, a skilled manual trader can beat every ea, because he can adapt to every market condition in no time.
also in order to write a profitable ea, you need to be able to trade manual profitable first.
so there is really no need to use an ea. often people say they have no time to trade manual and thats why they use an ea, however you can be profitable with a decent return with just spending 1 hour every week for your analysis, trading weekly e.g..
so manual trading is the best :). the only reason i can see many are using eas is cause they are lazy. just search google for an ea, attach it to chart and expecting to get nice returns with no work. unfortunatelly thats not how it works, even in life nothing comes without work, discipline, passion and desire.