Both solutions have their pros and cons.
The in-place migration is likely more suited for "smaller" sites where you want to preserve the existing configuration and content and when there are no changes in functionality, code or whatever. The main issue with in-place migration is the major pain in the a** factor in Plone projects called ZODB. The ZODB neither a decent database solution nor a reasonable database regarding migration. In a recent Plone 4.3 -> 5.2 migration we have high migration costs directly related to the ZODB. One part of the cost is related to an inefficient AT -> DX migration of Dexterity, the other part is related to the ZODB format migration. There is no offense included with this statement. I appreciated all work and efforts done here, in particular many thanks to Philipp.
An export-import migration is more suited when you have to reorganize your site anyway or doing large changes,
The overall problem (which is related to efforts and costs) is performance. I am currently helping with the migration of the University Ghent website. 90.000 content objects, 60 GB of data..some would call it "big data", I call this peanuts. Importing the data takes 36 hour - perhaps it might be possible bring the duration done with some optimizations. But out of the box performance without trickery is far away from being called "enterprise-level" CMS. Importing the same data (based on their JSON representation) into a document database takes 10 minutes.
So the decision of in-place vs. export-import migration depends on various factors. There is likely room for optimizations with the in-place migration and its complexity...but an export-import migration also is a complex operation.