Yjit is based on Maxime's basic block vesioning PHD work (a JS JIT), the approach both has taken is very dynamic-type-focused, it's a clever way to basically propagate type-info as the code is built. The main benefit is that you get a sane JIT fairly quickly and it should behave well in most dynamic typing scenarios.

They're pivoting (successfully?) to a more traditional way, letting the interpreter first profile the code (to figure out the types) and THEN produce entire methods with heavier optimizations that can do better register allocation.

The BBV approach is sane out of the box but kinda unfamiliar for many compiler writers (problems hiring?) and probably has some performance ceilings without much complexity.

The major question as to what method will win out depends on this question, how "monomorphic" or "polymorphic" is Ruby code in real life?

Monomorphic basically means that only one "real type"(from the compilers point of view) will ever pass a codepath (and thus extra machinery to allow multiple types won't bring much benefit).