As others have said, Wards are quite vulnerable on the inside, where their physical anchors can be damaged. As detonation of the equivalent of a nuclear bomb would certainly take out the buildings and everything else they are anchored to thus the wards would fall.
The boompit is a shortcut; to do all the blast of a nuke with magic you need thousands of shifts of power. But if you spend a couple hundred shifts not to make a big explosion but to harness the energy of fuel that when burned would unleash the energy of a nuke, then you can use magic like the spark plug in a fusion bomb. Just like Ebenezar used the already extreme kinetic energy of a big sattellite and Earth's gravity and merely directed it with magic (a 20-ton weight at 6 km/second has ALOT of energy) or when he used an existing volcano and just initiated the eruption with magic, so the boompit follows the same principle.
The drawback you already mentioned; it is possible to trigger the pit prematurely if you can get to it. A much bigger landmine spell that does the same without shortcuts can have failsafes built into the spell that are that much harder to tamper with.
