TYPO3 Core

The Installtool-Autoupdate never checks if symlinks can be created and therefore an update can fail with a non-working site.
Existing symlinks to index.php and typo3_src are deleted as part of the update and with the target to create them new. Depending on user-rights on the server there is no guarantee that existence of symlinks also allows creation of those. So an update can fail in the last steps when it's tried to crate the symlinks without success.

For the case that symlinks can't be created it's possible that it can be repaired without symlinks:
1) the symlink to typo3 still exists, then it's enough to rename 'typo3_src-x.x.xx' to 'typo3_src' and to copy index.php in the main-directory
2) the symlink to typo3 either never exists or still exists, but was wrongly showing directly to typo3_src-x.x.xx/typo3 (the older version of TYPO3):
Then the steps in 1) have to be done but additionally /typo3_src-x.x.xx/typo3 has to be copied to /typo3

For the case that FTP-Access never exists the installation is not working anymore and the update leads to undesired results and consequences.

If/When any test concerning symlinks in the install-tool will be included it would be nice still having the option to chose the options in the list above for the case that new symlinks can't be created.