I used to do proof of correctness work, decades ago.[1] We had more proof automation than many of the later systems. The easy stuff was solved by the first SAT solver, the Oppen-Nelson simplifier. The harder stuff used the Boyer-Moore prover, which uses heuristics and previous lemmas to guide the theorem prover. The Boyer-Moore prover had to be helped along by suggesting lemmas, which it would try to prove and which would then be used for later proofs. That was the tough human job.
Later systems seemed to have less automation. What tended to go wrong with formal methods was that the people doing them were too into the formalism. I was working on this for a commercial project that wanted bug-free code, not as an academic. The academic projects tended to get too clever. They had the mathematician's bias of wanting a terse notation and not much case analysis. That's not a good solution. You want lots of automated grinding and don't want to need much insight. The clever people kept inventing new logics - modal logic, temporal logic, etc, - to avoid verbosity in paper and pencil proof work. That wasn't really all that helpful.
The basic truth of this business is that most of the theorems are rather banal.
As the Jane Street people point out, there's a big advantage in having control of the language. You want the verification statements integrated into the programming language. In many systems, they're embedded in comments, in a different syntax than the programming language, or even in completely separate files. This adds unnecessary work. It's good to see them pushing this forward.
We were doing this too early, over 40 years ago. It took about 45 minutes of compute back then to build up number theory from a cold start with the Boyer-Moore prover. Now it takes less than a second.
[1] https://archive.org/details/manualzilla-id-5928072/page/n3/m...