While the BC doc states that Doctrine is now in use, what it does not make clear is that the ezcdbhandler interface put in place is now empty, and the DatabaseHandler interface which subclasses it does not expose all the methods that the previous handler had (thanks to its usage of magic method __call forwarding calls to the ezcdb connection object).

In short:
if a dev had used in his/her code the 'ezpublish.api.storage_engine.legacy.dbhandler' service, and used metods such as query() on the object received, the code will now break.

I have looked both in the BC doc file within the app itself, and all teh release notes online, and found no clear notes that this would happen.

Thanks for updating the docs so that future migrations from ezp 5.0-5.2 will not run into this