I'm a fan of scylla too, but if I could go back in time I'd have recommended waiting until mid 2019 to migrate. 'Fully optimized C++ rewrite's tend to take years to become battle tested.
there is no substitute to testing tho
there are 2 levels here. 1) raft has a proof (and a great phd dissertation from diego), but what matters is if we actually implemented it correctly. so 2) is we need to continuously test it. Denis did a lot of similar work at CosmosDB (microsoft) and has spent his career working on consensus.
Hopefully these eases some concerns.