That's right, it's all in implementation. Below grade construction has a lot of pitfalls particularly if someone cuts costs in the wrong place. Basements that are poorly constructed are a good example: walls bow and shift, they 'sweat' moisture, drainage becomes a real problem sometimes, etc. Sure, quality leak free below grade construction is possible, see Cheyenne Mountain, the various underground bunkers for ICBMs etc but look at what it costs.
Kinda gets out of the average Joe's price range.
The Quonset's arch probably would be strong enough if the bottom 'legs' were tied together to keep them from sliding outwards as weight was added. That means the slab or whatever would need to be very carefully designed, probably a prestress concrete slab with the rebar placed so that the ends of the arch could be firmly attached directly to that rebar. Plus, the fill dirt weight would need to be added evenly.
