>>366857731
It does unexpected things or limits you. With write defaults off you have to push all the shapekeys or whatever you animated back to the default state manually, so while it seems tedious at first on paper it actually allows you full control over the state. As such, most prefabs you get that do things probably use write defaults off. When you have On set anywhere, it sorta forced EVERYTHING to work as if it's on so that's why it's bad, mixing write defaults causes issues.

If everything you did is all on Write Defaults On there will technically be no issue but you are probably not doing much to begin with,