Developers are likely to run into trouble in their migrations with references between apps. It must be clearly documented that 'reference' properties will lead to broken behaviour if an M6 CT-based app attempts to reference an M5 CT-based app.

It should also be documented how they can work around this by adding a link field explicitly in the app definition (not in the content type definition.)

For details, see: https://jira.magnolia-cms.com/browse/MGNLUI-6044