I think…
I think most say the current Subaru rack is not good in that it is not as strong as aftermarket racks (few manufacturer racks are) but it also isn’t easily replaceable by aftermarket.
Using the cross bars for a single kayak likely is fine. Sounds like you wouldn’t want to use for 2. And definitely not a stack of them.
Using a bow line or both bow and stern lines is a good extra precaution.