I have seen some darn ugly welds and joints on box store bikes - Huffy comes to mind in particular, as they used to have dropouts connected by crimping the chain and seat stays and spot welding.

Raleigh Canada made a lot of cheap steel frames for box stores, sold as Raleighs, and as Triumphs, among others. These had quite possibly the ugliest welds I have ever seen on a bike, but I do not recall seeing a failed frame, aside from some needing the rear triangle aligned.

But Raleigh doesn't make any of their own bikes now. All the box store bikes are made by contract manufacturers in Asia, and they are not great, but probable more than strong enough for regular riding. I believe they use cheaper materials but with much thicker tubes to make up for the material's lack of strength and for less precise QA during production... imagine a Giant brand aluminum frame, with engineers skilled at structural analysis and welding processes able to have strong frames made with thin walled, differentially butted and profiled members... now compare that to a company producing box store bikes - they will use much thicker aluminum to make up for the less-carefully welded and shaped tubes... the frame might weigh 2X or 3X more, but has the same likelihood of failure as the Giant.

Some things to look for that are indicative of poor quality are one-piece 'Ashtabula' cranksets and bottom brackets - these have never been put on good quality bikes AFAIK, and frame dropouts that are just cut or stamped from a flat plate, compared to dropouts that are forged or have an otherwise more complicated shape.