In a series of YouTube uploads, Seth from Seth’s Bike Hacks spends the $140.00 to test Walmart’s Mongoose MTB line and the results are what you’d expect. Not sure what Seth’s point is, but for some perspective, Walmart and Mongoose move millions of bikes a year and that’s quite a bit of bike for $140.00, and not really intended for the abuse they give it. These bikes are bought as gifts for kids or transportation to and from work. Also, see Chris Akrigg putting a Mongoose through way more abuse than Seth.
You get what you pay for or view on YouTube. So are Walmart mountain bikes safe? If you double-check the assembled and don’t jump them, then yes.