Only because Luffy his attack destroying multiple buildings, it doesn´t mean his attacks are building level. Physical laws in fiction are working a bit different from reality. Luffy his attacks are building level can be argued already before timeskip and after that he get a lot power ups, beating enemies who have a far higher durability. He overcome in PTS the Tekkai from Bruno, Lucci, etc. Later he crushed a Pacifista, which are known for her "Harder then steel"-alloy and after that he get Koka, etc. It doesn´t matter what his attack destroy in the area, when Luffy already beat enemies which are superior to the durability of a building.
A reason why that doesn´t look impressive is probably, because a lot of fiction characters can concentrate their power in a small area and attack the enemy with full power. You don´t see that it destroy mountains, since the enemy take all the force and not the environment.
Someone like Son Goku in SSJ3 punched through King Kai his small planet, which should be in mass not even mountain level. To say SSJ3 Goku has only mountain level punches is insane, since we ignore the scaling that happened before.
Another example is an explosion. The size of the explosion is not necessarily relevant. The explosion can be 3-4 bigger then a nuke and is still weaker, since Range<DC.
To put something on a level or above that, we need scaling and statements. Feats are oc the best way to proof something, but a lot can be only explained in a context with a statement.
Of course a character who have mountain level DC or a Striking Strength on that level, hasn´t automatically a durability on the same level. A lot of characters in fiction failed at this point, that´s what we called glass canon. Someone has far superior stats and abilities, but lacks complete in durability.
Luffy his King Kong Punches are multiple building level, when he already punched in base something that can be argued with the durability of a diamond and deformed it? Not the range of the attack is relevant, the scaling at this point is it what he did before. You can have a mountain level durability and Luffy would oneshot you with a KKG.
At least that´s how it is on other crossbattle boards.