I used waxoyl for years on classics albeit in cavities; as a general chassis or underbody wax protectant it isn't particularly durable on its own. It helps to preserve the life of older underseal, by keeping it supple as long as it's sound. However, old underseal on its own can crack or lift, and is worse than no underseal at all as moisture penetrates, gets trapped resulting corrosion under seemingly sound underseal.
However, I too have progressed to Bilt Hamber products, which in my opinion and experience are far superior to any 'named' waxoyl and similar rust prevention wax. They have different formulations for under body and chassis, cavities etc. Bilt Hamber dries to a usuable consistancy a lot faster than Waxoyl, and is less prone to dripping all over the floor after application.
Thumbs up from me for Bilt Hamber products without question.