The fact that BW is open-source allowing the ability to self-host is a very awesome and unique feature. The fact that Dani Garcia ported the code and allowed you to host vaultwarden on a low-power device like a Pi or a small VPS is even more awesome. The fact that they both made it easy to install and run the service with Docker etc., and that there are a lot of guides on how to set the whole thing up is super awesome. You can play around, learn some things, and get control of your own data. It's all awesome. But none of that is a security feature.