Hasura[1] is a PaaS (based on Docker+Kubernetes) + BaaS. There are some built-in backend components (like auth and a data service). You can also write any code and deploy (via docker images) to multiple clouds.
Disclaimer: I work at Hasura.
[1]: https://hasura.io
Depends on what you consider "unopinionated"...
It handles all the deployment lifecycle across all providers but won't abstract away all the implementation details. It is not possible to "lift & shift" from one provider to another without making some changes.
At the implementation level, the providers do differ. From small things (like the function interface) to bigger things (like event services).
I did a talk on this a while ago: https://speakerdeck.com/jthomas/taming-serverless-dragons-mu... https://serverless.london/meetup/feb2017/serverless-framewor...