I don`t get how your unit brakes fail in that instance either unless you mean you had no pressure so how can you pull it round in the first place.
For the OP split coupling is fine as long as you check the trailer brake.
It can be dangerous if you are a forgetful, lazy or stupid person in which case if the trailer rolls you could be crushed instantly or worse still I could be walking behind the trailer and it rolls or you shunt it on top of me.
Problems with both rolling away usually come from not applying the brake on your unit.