Hull loading
I think that double-bubble cross-section is rather popular. The upper and lower frames as well as deck beams in pure tension (with cabin and underbelly equally pressurized), no bending forces anywhere.
There are perfectly circular planes, sure. Airbus 300 series as well as Boeing 777 series.
The advantage of double-bubble is that thanks to pure tension, it is the cheapest way to deviate from circle. Boeing 767 is strongly double-bubble while 777 is not. Reason? The underbelly bulge ensures that 767 underbelly has about as much headroom as 777 underbelly, even though it is narrower. 777 is wide enough that there is adequate headroom for both cabin and underbelly in perfect circle, so a perfect circle it is.
The narrowbodies again are double bubbles in order to get extra underbelly space. Airbus 320 is wider than 737, and very slightly double bubble compared to 737 - but it is not a perfect circle.
380 is triple bubble - again, so as to get the headroom in underbelly and main deck and upper deck.
But the ends are problematic. As are doors.
I think Boeing has had one disaster uncontroversially because a cargo door failed... it was in underbelly - and when it failed, the pressure in cabin broke the cabin floor above, blowing 9 passengers with their seats out of the plane! The rest of the B747 kept flying, though, and eventually landed.
Douglas has had a rather similar disaster - cargo door on a DC-10 failed, 6 passengers above blown out with seats... but the rest of the plane crashed as well with over 300 fatalities.
Well, passenger B747 do not have nose door to blow... but if a B747 nose door slams into the cockpit on climb, what are the chances the plane crashes with those aboard and a lot of people on ground?