The installation of an add-on including its dependencies is no longer transactional

[zopyx]

The fix for

#2228

introduced a new undesired installation behavior.

The installation of an add-on (and its dependencies) in Plone in all versions has been transactional until Plone 5.1.

Plone 5.2 introduces a workaround for the bug mentioned above which gives up this principle.

As clearly documented in

#2228 (comment)

the new behavior can bring a site into an unwanted and undefined state. An installation issue with a package must never ever install any further dependencies. A rollback of the transaction for the add-on itself and its dependencies is the only valid option in this case.
Full success or full rollback. Nothing inbetween.