I know it's balance and all but my pet peeve is shambling, brainless zombies being able to burrow through reinforced concrete and steel plate with their bare @%$#ing hands in seconds or minutes. That thing should be almost indestructible. I'm all in for making it either super expensive or super time-consuming or both but when I finally get to the point when I can improve my base with that stuff, I expect it to easily last an entire horde night, even on the highest difficulty/highest block damage setting.
As it is right now, end-game base building materials are just so inadequate and flimsy.