Tung oil is not the best finish for brushes, but it's still a lot better than no finish at all.
IMHO the best finish for brushes is a hybrid oil/topcoat. an example of such a finish would look like this:
- oil, several coats, burnish between coats
- lightly sand
- shellac wash coat
- polyurethane (or other waterproof topcoat)
By using oil and then topcoating, you greatly reduce the chance of water infiltrating through the topcoat (either physically through cracks/scratches/dents or by means of vapor transfer, which is what happens when liquid water is allowed to sit on it)