Microsoft Office is frequently updated to patch security vulnerabilities (CVEs). A portable version is a "snapshot" in time; it cannot be updated via Windows Update. If a critical security flaw is discovered in Word 2019 next month, the portable user remains vulnerable indefinitely, creating a security hole on every machine the USB drive touches.
If you need Microsoft Office features on the go or without a full installation, use these official methods: : Free to use in any web browser. Includes Word, Excel, and PowerPoint. microsoft-office-2019-portable