I decided to get rid of my old versions and add the latest LTS:
If you were to do a clean install of Windows, I‘m 99.99998% sure it‘ll work. 
Meaning the culprit here is always something specific about the system, not the installer, otherwise nearly everyone would have these issues.
Common causes include antivirus or similar resident, intrusive applications (malware) or an abnormal OS configuration (personalized user or file system permissions), missing updates and/or a compromised system (including manual or automated „registry cleaning“), or driver/hardware issues. Or following bad advice such as „run as administrator“.
The best you can do is look at the logs (locations in the manual) to see if they are at least hinting in a certain direction. Cleaning the cache locations can also sometimes help, it can‘t hurt to do so. And ensuring the OS has the latest updates and driver versions.
UPDATE: “Validation failed” occured to me recently. It was due to not responding to the UAC prompt because I left the computer after the download started, and the UAC prompt eventually gets dismissed after a timeout leading to “Validation failed”. Simple retry and confirming the UAC prompt fixed it.
No thanks to Unity Hub for vague errors without a hint to where the logs are, I found them at %appdata%\UnityHub\logs\info-log.json
{"timestamp":"2024-02-22T06:28:51.596Z","level":"info","moduleName":"UnityInstallerWindows","message":"[ 'spawn child process, elevated:', true ]"}
{"timestamp":"2024-02-22T06:28:51.596Z","level":"info","moduleName":"UnityInstallerWindows","message":"[ 'executing', '\"C:\\\\Program Files\\\\Unity Hub\\\\Unity Hub.exe\"', '-- --winInstaller' ]"}
{"timestamp":"2024-02-22T06:28:52.247Z","level":"info","moduleName":"UnityInstallerWindows","message":"[ 'user did not agree to the UAC prompt and the process was not started' ]"}
{"timestamp":"2024-02-22T06:28:52.249Z","level":"info","moduleName":"UnityIPCServer","message":"[ 'IPC server Unity-hubInstallServer closed' ]"}
{"timestamp":"2024-02-22T06:28:52.249Z","level":"error","moduleName":"UnityInstallerWindows","message":"[ { code: 'User did not grant permission.', message: 'The Hub does not have proper permissions to install Unity.' }, undefined, undefined ]"}
The logs state that I did not grant UAC permission, yet the UAC prompt never came up. Specifically:
Unity Hub.exe\"', '-- --winInstaller' ]"}
The Hub does not have proper permissions to install Unity.' }, undefined, undefined ]"}
…likely due to undefined, undefined. There’s also the strange “-- --winInstaller” flag, where a —flag seems to be partially added but missing. I’d almost bet money that there’s some kind of --elevate flag that’s supposed to be there. Heck, it probably also detected a bad-named flag named either " " || " --winInstaller" since it’s probably detecting the next word after “–” which !exists.
Hub has a track record of having poor-quality updates - if Unity was Conanical or someone trusted to actually have proper QA testing, this would be different. However, we’re talking about Unity, here, infamous for lacking QA - with Hub intensifying this statement tenfold. I doubt it’s me.
The two undefined vars on top of the missing --flag seem to scream “we were supposed to elevate, but an untested Unity Hub bug prevented it”.
