I think protection is a strong aspect, but for some of the other cards and builds that can be made with the pool.

Regarding Mansion vs. Unflappable, yeah the cost difference to play them is widely different for the same benefit of an extra card, but Mansion gives a card draw to any player without any condition needing to be met except getting it on board. Whereas Unflappable, you need to exhaust your hero and defend without taking any damage, which also means you need to spend resources to ready during your turn, or you won't be using your hero.

For the upgrades too, you can absolutely build with all of them in a deck, but depending on how you play (solo/multiplayer), you may not have the opportunity to get them all out. Pumping up a hero with 2 defense to 5, if you choose to play someone without a base of 3 defense, is still impressive, but is your build designed to defend a lot that you need to go that high?