The simplest JIT just generates the machine code instructions that the interpreter loop would execute anyway. It’s not an extremely difficult thing, but it also doesn’t give you much benefit.
A worthwhile JIT is a fully optimizing compiler, and that is the hard part. Language semantics are much less important - dynamic languages aren’t particularly harder here, but the performance roof is obviously just much lower.
Agree re: different types of JITs producing wildly different results but don't agree about language semantics - even a Java JIT has to give up speed due to certain seemingly minor language and JVM issues. So both matter - no matter how good of a compiler engineer you are, some semantics are just not optimizable. Indeed, the use of a "trace JITs" is a proof of that.