From what I understand, that's the biggest problem with using UBI as a replacement for wages/salary. It's probably better than no social safety nets, but ultimately its' buying power would be greatly diminished.

My feeling is that we probably won't see a 40-50% unemployment rate when automation starts up in earnest, people will just find new fields to move into. This isn't our first technological leap forward, and if we look into the past, technological leaps can actually be beneficial to job creation. The Digital Revolution eliminated tons of (mostly menial) jobs, but created so many more in the process. I don't personally believe we have to be afraid of automation.