I suspect that slopes are like strong head/tail winds - the advantage you gain one way is not as great as the disadvantage the other. I refer to head winds slowing you down so much that turning round and using the tailwind does not recover all that disadvantage.
A bowl shaped runway - does the upslope on the second half have a greater negative effect than the gain in the first half? If there is more down than up, how bad is that for landing (bearing in mind that weight for weight you need more runway to land than take-off).
A crest shaped runway will give a nice boost later but will have hit you hard initially.
Building runways that are good in one direction means that they will be bad in the other.
As I say - I don't know the answers, these are just speculative thoughts.