SSD chips do go bad over time; most vendors recommend total replacement after 240 gigabytes of overwrites per gigabyte of chip. Soldering down the SSD flash chips means these machines need Ball Grid Array rework when reconditioned for resale after lease return. Apple will be doing that rework, so we can maybe assume the ticklish microsurgery doesn't phase them?
Why solder them down in the first place? Because of the T2 chip. T2 is very…possessive of SSD memory.