several.. truth is that whatsapp was made to be used in one device only and is not frindly to be used in multiple devices at the same time.
Until recently (like days ago) you still needed the main device to be online and connected to their server to use the both the web or desktop apps, although for some time there was a beta going for the feature they released a few days ago where that is "no longer required".
Even so, i was on the beta of the new feature where you could use the web or desktop app without the phone since it started and it has its quirks.. Often it fail to load either the history or even new messages, messages sent on the main device often do not show in the web\desktop app and performance is overall terrible.
Also there is still no official way to have the same whatsapp across multiple phone\tablets devices.
