For what it’s worth I have had a flying blue account for several months having never credited a flight to FB before. I transferred ~140k points from both Amex and chase and booked two business class (operated by delta) tickets for myself and my wife with no problem, the ticket are ticketed and confirmed, I have a PNR for DL and can change seats and everything.
i can’t speak to if these are replicable results or if higher transfer amounts trip anything but my experience went fine.