Before following this tip, review the bootstrap folder and strive to determine why the upgrade failed. If you cannot determine why the upgrade failed, the advantage of reinstalling the operating system (over using msiexec /x) is that you have ensured all remnants of a failed and in-solvable upgrade are completely removed.
By following this tip you will not gain any assurance that you have a clean upgrade. It you 'must' follow this article because there are non-SQL Server processes installed on the same OS that also depend on SQL Server, by following this tip you may end up with new problems in those processes. And if you call MS Support with a new problem at any time in the future, and if they discover msiexec /x had been used to blast away an old version, you may end up hearing Supprt asking you to reinstall the OS.
In other words, msiexec /x is not supported by SQL Server Development or SQL Server Support, and the consequence of using it (such as abnormal product behavior) is also not supportable.
Caveat emptor. It is far better to flatten the box, IMO...