CephFS implements a (fully?) POSIX filesystem while it seems that TernFS makes tradeoffs by losing permissions and mutability for further scale.
Their docs mention they have a custom kernel module, which I suppose is (today) shipped out of tree. Ceph is in-tree and also has a FUSE implementation.
The docs mention that TernFS also has its own S3 gateway, while RADOSGW is fully separate from CephFS.