The most common files are documents, videos and photos.
I sync accross android and windows peers. I didnt buy the 1621+ yet, the 1512+ works really well since almost 9 years, but became a little bit slow now.
But when final time for change comes, i will write down all permission keys to be sure.
Alternative solution is Synology Drive, works well on windows and syncing to another nas, but the android version isnt that good and does not sync at all.