Good for hobby or personal projects, but not if you’re building something for work. Many of these “free” tiers are designed to vendor lock you before they walk you off a pricing cliff.
Take Auth0. Free for up to 7,000 user but then it jumps to about $250/month. Migrating from Auth0 is a pain. You should really only consider these “free” services if you think you’ll always be in the free tier, and if you’d use the service even if it wasn’t free.