The info that you know requires more privacy remains in the local KB. The info that you really want to access and sync from many devices goes in the synced KB. And all the rest (99%) has to choose sides. And commit to it, because there’s no robust and easy way of moving pieces of a KB from one to the other, not even manually. You can’t reference synced content from your local KB even if that doesn’t present any concerns and it all exists locally anyway.
So, for example, I’d like to be able to control which parts of a knowledgebase are synced and which are not, and change that on the go. Or at least, I’d like to be able to export robustly parts of a KB and import them to the other.