A Flat Overhand is not as strong as a Double Fisherman’s knot to tie a cordelette in a loop. Some climbers will still favor a Flat Overhand over a Double Fisherman’s because it is easily untied, and the cordelette can be used as more than just a loop (e.g. rescue). A Double Fisherman’s weighted several times may become very difficult to undo.
The Flat Overhand is also known as the Euro Death Knot, supposedly because American climbers didn’t trust it when it first showed up on the new continent. Although the EDK name stuck, it is now widely accepted everywhere to join two ropes for long rappels.
The forces sustained by the leg of an anchor during a fall can be much higher than during a rappel. Several amateur testers (1) and even a French official testing facility (2) recommend not using the Flat Overhand to tie a cordelette in a loop because it fails by rolling under relatively low loads. All those tests are done with slow traction.
The Centro Studi Materiali e Tecniche of the Club Alpine Italiano showed that the knot performs much better in drop tests than under slow traction. (3)
The cord material (e.g. Nylon vs. Dyneema), the type of anchor (e.g. two or three legs), the tails length, and how well the knot is dressed and cinched are important factors.
Although a cordelette with a Flat Overhand is safe if used correctly, less experienced climbers should stick to a Double Fisherman’s for tying a cordelette.
(1) https://user.xmission.com/~tmoyer/testing/EDK.html – https://canyonsandcrags.com/pdf/which-bend.pdf