When recommending for or against any of the inexpensive bicycles it's misleading to state all of "X brand is just fine and do not break" or even "aluminum is bad/steel good" as it's unfortunately far more complex than just brand name, price or even material as the quality control is highly variable.
Even higher cost bikes suffer some variance in weld quality, the difference being someone most likely inspected them before letting them out of the factory - with the cheap bikes, you'll have to do this yourself.
Judging from this;
I think the pics are turned 90°? ...but, I'd hafta say it looks like the welds were not only hurried (wide lap spacing) but also varying in types of problems. While the seatpost to top tube weld held - it's obv too fast w/too low a current, whereas the weld that failed (stay to seatpost) was too slow/current too high & thus undercut - which leaves a space/void where there's less material & that's where it most likely initially cracked, the clean break in the stay probably came after as a result of the added stress loads.
It's easy enough to inspect welds if you know what you're looking for - remember they're far
more important than brand favoritism, material bias or shopping by price tag alone as they tell you clearly how well that particular frame was built;
Common low quality welding, steel & a 'NEXT' brand mountain bike;
Beautiful welds, aluminum & a 'Schwinn' brand mountain bike;