Self hosting BitWarden still means it's accessbile for them and/or from them. You also have no way to audit their security from what I understand. VaultWarden is FOSS, if you want to, you can go check. And it does get checked by people with the competence to check this do every now and then. [Edit: I forgot that BitWarden is actually souce-available as well, while not being FOSS that's still better than most solutions]. I just prefer full FOSS whenever possible. I prefer it not be a black bos I just happen to run on my own server.

If you self host VaultWarden, the instance can just be not accessible from the internet, and only from behing a VPN. Obviously this is inherently much safer. If that's possible with the self-host option I don't know, but even just for licensing the local instance will have to be able to reach their servers (possibly be reachable from their servers, too). I did see they got an "offline deployment" option for air-gapped servers, but haven't looked into what limitations that entails.

Additionally, you're still within their licensing model. So for certain features you need to have a not-free account (like even just more than 2 people).

And like others said, VaultWarden is much lighter on resources in general and you aren't limited in what you can and can't do (users, collecitons, auth-options, ...).