The biggest issue with using WBP over epoxy is warping of the wooden structure.
When using WBP over raw wood it will warp & the poly will cure before the structure can be straightened therefore locking in the warpage.
So sanding sealer is used to reduce the amount of soakage into the wood, which also limits how much grip the WBP has.

It is true resin is far harder & bonds stronger, but it is heavier than WBP, marginally.
Both products should be thought out prior to using, & there is no rule that says you can't use both on one plane.

I have a kit that has thin wing skins which will make it dead easy for some noddy to squeeze to hard & fracture/pierce, so I'm gonna use resin to lay up the wing, & WBP for the fuse which is a far stronger stucture anyways.

Like anything, there are ways to save weight, or cancel weight out in other areas, logic applies.