Opinions on Synthetic Users for Research?

Hiya. Im a user design researcher who has been working in the field for roughly 4 years. Today I read about synthetic users being used in research to save on costs. What's your opinion on this? Have you personally used synthetic users in your research? How did it go?

I'm of the mindset that I would never use synthetic users as I don't see how it would generate new insights, but happy to have my mind opened.