Syncing seems to be fine. The folder structure in Classic doesn't get migrated to CC unless you create collections before migration. Because I didn't do it at the time, I tried creating new collections in Classic after migration to see if they would be organized into albums in CC but it seems like this is not possible after migration? Seems like a lot of people are having the same issue. It seems there is no easy way to recreate the folder structure unless you manually create albums which is just too tedious when you have >12,000 images.
Regards
Sawan
As I understand it, after migration the Classic catalogue will no longer sync, so you would have to create a new catalogue, enable sync with it, and then you would see collections interchanging with LRCC.
What would also happen is that the new catalogue would start downloading originals from the server. You may not want that.
While there is some communication between LRCC and LR Classic, Adobe haven't said they are intended to work together and haven't designed them to do so. Naturally enough though, people imagine they should build such a workflow, but I wouldn't recommend it in general.