Google is improving security of Google Play Store step by step. In the past company removed most of apps from unknown sources, but how shows history with fake version of WhatsApp we still can't feel completely safe after using all apps from Google Play Store.
Google didn't spot the fake version of WhatsApp for a long time, and over one million of users downloaded app named "Update WhatsApp Messenger" which even had the same source as the official version of WhatsApp. Creators of fake version made it with using official logo of WhatsApp, and they changed Unicode space which caused that programs which are checking security of new apps in Store haven't found dangers. However, in fact fake app wasn't too dangerous because the purpose of the creators was only making money from advertisements.