Their technique (by-method JIT, LLVM toolchain) is more likely to produce a mature implementation in the short term, possibly with a modest (but welcome) speedup, without major incompatibilities with extensions. If it happens, it's better than "That Perfect JIT VM" that is never finished.
The major issue with PyPy is that it has a radically different C API than CPython, leading to incompatibility with NumPy, Pandas, etc. to mention just a few prominent performance-critical Python packages that are written in C. PyPy works if you want to use pure Python code, but that's rarely the case in real-world high-performance Python projects.
Compatibly with CPython's C API will always be a problem for any other implementation. The only practical solution is to switch to a better FFI, like cffi.
