I've tried Avivo with the old Catalyst 9.7, and I wasn't impressed by the quality. It may have done things in 15 mins as compared to about an hour on a cpu, but the quality on cpu based encoders is still better (though it takes time). I don't know about the latest version of CCC though.
On the nvidia side of things Badaboom is the leading converter, though it isn't free and the options you could change are a bit limited.