NGINX requires external tooling (e.g. certbot), Caddy's auto-HTTPS is built-in. Caddy's auto-HTTPS logic is much smarter and more robust than certbot or any other ACME client. In general, Caddy sites stay up when other sites will go down due to HTTPS/certificate issues. Certbot+cron is much more brittle than Caddy's embedded handling of certificates. Plus, Caddy has stronger memory safety guarantees than a C program like NGINX.

Caddy automatically staples and caches OCSP responses by default, as well. OCSP responder outages? No problem. This sometimes brings sites down (esp. Must-Staple certs in Firefox) because other servers' OCSP stapling implementations are not as robust. I remember when gnu.org went down for a while because of this.

Certificate got revoked? No problem, Caddy will automatically detect that via OCSP and replace it for you.

Internal throttling and retry logic makes it more resilient to domain validation problems. Caddy will fall back to a secondary CA if your first CA is having trouble issuing a certificate (i.e. multiple default CAs). Not to mention, when using Let's Encrypt, Caddy will retry with its staging endpoint to help avoid rate limit issues.

Plus, deploy multiple Caddy instances as a cluster and they will automatically share and coordinate management of certificates and OCSP staples. (Doing this is as simple as configuring each Caddy instance to use the same storage backend.)

It's very quick to get started and try out HTTPS: https://caddyserver.com/docs/quick-starts/https