Authentication is the feature every engineer is sure they can build in a weekend, and they are right. You can build a login form, a users table, and a password-reset email by Sunday night. The weekend is real. The weekend is also not the cost. The cost is everything that arrives after it, on a schedule you do not set, for a system you can never turn off.
The part you think the job is
Email and password. A sessions table. A "forgot password" link that emails a token. Social login if you are feeling thorough. This genuinely is a weekend, and if that were the whole job, you should absolutely build it. It is not the whole job, and you know it is not, which is why you are reading a build-versus-buy article instead of just building it.
The bill you don't see coming
You now own a security surface, permanently. Password hashing, and re-hashing every user the day you realize your bcrypt cost factor was tuned for 2019 hardware. Rate limiting and account lockout. Credential stuffing, because your login endpoint joins every breach-replay list within a month of launch. And the big one: liability. The day you store credentials you become a target, and a breach stops being an abstract risk in someone else's pricing deck. It is your incident, your disclosure email, your customers' trust, and possibly your regulator.
