I just installed the new public beta of Publisher and the first thing I tried to do was to open an InDesign file. Affinity Photo and Designer both do good jobs of opening PSD and AI files so I was expecting the same from Publisher. I've tried opening INDD, IDML and XML files exported from InDesign but none of them are recognized by Publisher.
I can still open up InDesign to access the INDD files but anyone who didn't get on board with Adobe's subscription service (like myself) will be running an older version that's already got barely limping along on the current OS. I have no idea if it will work at all under the upcoming Mojave OS.
I have completely switched over to Affinity Photo and Affinity Designer and even though opening PSD and AI is sometimes not perfect, it's certainly close enough. I never used InDesign nearly as often as Photoshop of Illustrator but I've still got files created over nearly two decades (the early ones converted from Quark) that I may need to revisit at some point.
It would be nice if a representative from Affinity would weigh in on this and let us know if there are plans to include the ability to open INDD files in Publisher.