Non of the above will fail. The point of enclosing it is so the ferrite does not get wet. You do not want the properties of the ferrite to change by being wet. If you took the wound balun and dipped the whole thing in plastidip, or use conformal coating or potting compound, then it would not need to be in a case. However, you would need to ensure that they do not break down over time with UV etc.+ the mechanical considerations listed by @KK5JY Easiest thing is to stick it in a box, grommet in, grommet out and a weep hole for condensation.