That depends on the individual. Some FAANG employees are used to functioning in the highly structured environments that FAANGs provide and may not adjust well to the more freewheeling environments that startups operate in but others may adjust just fine.

That's not to say that people who function well with a structured environment aren't useful. Structure is necessary as the business grows out of the start-up phase and bringing in the right ex-FAANG employees at that phase to can help build that.