So far they are:
Inappropriate drive reassignment can occur on eligible computers that have an external USB device or SD memory card attached during the installation of the May 2019 update. For this reason, these computers are currently blocked from receiving the May 2019 Update. This generates the error message that is mentioned in the "Symptoms" section (of the above article) if the upgrade is tried again on an affected computer. The drive reassignment is not limited to removable drives. Internal hard drives can also be affected.
2. An old version of anti-cheat software - Some game developers, publishers, and digital distribution companies that run online stores for game downloads, include anti-cheat software.
It has been suggested that before upgrading to the May 2019 update, make sure to update anti-cheat software. Windows Upgrade process will check the version, and if the version is old, the upgrade will fail.
3. Redirected Known Folders - Known Folders are folders offered via OneDrive to make sure essential folders like Desktop Documents and Pictures are always backed up on the cloud. However, if you have redirected any of the Known Folders, an empty folder with that same name is created in your %userprofile% directory when you update. Actual user files are not deleted. A fix is in progress.
4. AMD Ryzen™ or AMD Ryzen™ Threadripper™ configured in SATA or NVMe RAID mode
Certain versions of AMD RAID drivers are not compatible with the Windows 10 May 2019 update. If a computer has these drivers installed and configured in RAID mode, it cannot install the May 2019 update of Windows 10. If you start the installation process, the process stops.
Version 126.96.36.199 and later versions of the AMD RAID drivers do not cause this issue. A computer that has these drivers installed can receive the May 2019 update. You must install updated drivers in order to resolve this.