Time Machine by default backs up nearly everything in your macOS partition. It excludes log files, Spotlight indexes, caches, temporary files and trash (have a look: On OS X, what files are excluded by rule from a Time Machine backup?). The backup includes docker files, npm packages, your personal files etc. When the backup is completed, the hard drive will NOT be bootable, meaning that you can't run macOS from your Time machine backup.

You can restore from a Time machine backup without or after installing an operating system. If you restore without installing an operating system:

• All the backup will be restored to your disk, including the operating system (for example Yosemite). [NOTE: If the Mac you are restoring to does not support the version of macOS in your TM Backup, it will still not be able to boot from the restore].
• It can be done from either Recovery or Installation USB. No internet access is required.
• After the backup is restored, everything will be exactly how it was while you were doing the backup.

If you restore after installing an operating system:

• Only personal files and the files you select can be restored and will not restore the operating system.
• Might not restore packages, system tweaks and files that are stored in directories other than traditional ones (for example "/MyApp/mysourcecode.cs" will not be restored by default).

If you think this contains misinformation, please comment because I never actually restored after an operating system installation.

I have done this and it works. Open the mac in recovery mode. Wipe your hard drive and select restore from time machine. The OS and your files will be restored.

You can't. TimeMachine in 10.11 won't read files created by Time Machine in 10.12. That's why Apple's first comment is most betas is that you can't restore a previous version OS.

This doesn't mean you can't erase and reinstall the old OS, it means you can't restore your files which you created in the new OS to the old OS.

It's no surprise that Migration Assistant won't help you with this.

You can use the Time Machine UI (in the Finder) to restore your users' home directories.

For most apps, you'd be better off re-installing them from their installers, especially if they might not be self-contained. For example, apps sold outside of the App Store often come with installers that install a bunch of stuff in various places under /Library, and it would be hard to find all those files to restore them to your High Sierra environment.

For system settings, just set up all your settings again by hand.

The problem you're facing is that things like how settings are stored, how user account information is stored, how your Photos.app and Mail.app data is stored, how your Spotlight search metadata is stored, etc., often change from one major OS release to the next.

Apple writes the code in the new OS version to upgrade those settings and data to the new formats as applicable, but it's not cost-effective for anyone to write code for the old OS version to know how to downgrade settings that have already been migrated to the new version. It's just not a thing that enough people need to do often enough to be worth the software engineering work.

Beware that for built-in apps like Photos and Mail that store their data in proprietary, undocumented database-like formats, you might have problems downgrading. For example, if you already launched Mail or Photos under Mojave and those apps already updated their databases, then when you try to use the High Sierra versions of those apps, they might not know what to do with the new format and may just fail to work with it or may corrupt it.