Apparently the S model was more resilient to the solder joints breaking, but still had a somewhat inadequate cooling solution. That article goes into more detail, which basically comes down to Microsoft tried to save money by doing all the design in-house, and ended up botching the whole thing, under testing it, and doing last minute changes (like the addition of a Hard Drive, which modified the airflow negatively, so they put some extra holes in it, but ultimately said “fuck it, ship it out”.