It's about 80% easy to share an object such as a logo or ad design or similar between the two packages, which is facilitated by the programs sharing an overlapping feature set.

With some features, such as gradients (InDesign to Illustrator) and type (Illustrator to InDesign) there are serious problems and the feature either doesn't travel across or is corrupted in the process. These problems mean that some parts of your project will easily copy and paste (or travel via PDF export) yet others fail.

It's not uncommon to use Illustrator as a publishing software for web assets and InDesign for print and use them as partners for that, so let's see if the overlapping features can fully travel between them, saving workflow and increasing confidence in the term 'Adobe Suite'.