The Google Play Store checks your device specs and may block the installation of WhatsApp on older tablets or devices without SIM cards. The APK method bypasses these hardware checks, allowing you to install WhatsApp on devices that the Play Store has marked as "incompatible."

This is Uptodown’s killer feature. Sometimes, WhatsApp updates contain bugs, UI changes that users hate, or features that break compatibility with older Android devices.

(It is as safe as a third-party source can get).