WhatsApp is closed source, so you have no idea what it's doing. And they can push an update doing whatever they wish to you at any moment. You have to rely on Facebook pinky-swearing that it is what they say it is. And I promise you it isn't what they say it is.
Being open source isn't necessary or sufficient to be able to audit a piece of software. Software can be audited even if it's closed source (for example by reverse engineering, although it's more difficult), and even if it's open source it might still be impractically difficult for non-experts to audit.
