Is there a significant difference between this and nginx proxy manager?

They're both reverse proxies built on nginx, but the whole point of BunkerWeb is that it's a WAF, which NPM is not, so that's a significant difference.

In short, NPM doesn't do any of the stuff listed under Security Features here: https://docs.bunkerweb.io/latest/#security-features

NPM will automate Let's Encrypt certificate generation but you're right about the other listed features.