I have to say, I agree with the engineer.. I'm cringing just knowing this sort of practice is seen as acceptable, and it reinforces my distaste for Java. If you have to resort to insecure practices to achieve performance, there's something wrong with the framework.
Of course you have to resort to low-level functionality to achieve performance. That's how computers work. We can't all live in fuzzy wuzzy land.
Surely Unsafe is safer than writing the whole thing in C and using JNI?
(separately: why the downvote? I get downvoted for having the opinion that it's worth a tiny bit of performance for improved security?)
But that's not a reasonable definition of "unsafe". This could crash the JVM is a reasonable definition. And fences and ordered instructions cannot.
As far as use in libraries, you wont get the same performance without it in most of the places it is used. This is much like Python's 'unsafe' use of C modules for performance.
As another reply pointed out, much better to do some hinky things with the JVM then to call out through the native interface every time you need high performance for a tight loop.