[rurban]

pypy changed not only the vm, but also the internal data structures. It's totally different. pypy's problem is that is only optimizes loops and often called functions, not the general interpretation, as e.g. PHP7 or lua did. For normal, simple small scripts it's not that big a benefit, and you loose all the extensions, (though that's being worked on).

[acdha]

That has been less true for the last few years: C extensions are still a concern, although many of the major ones work now, but I've had a significant number of programs run faster simply by using pypy. It's a full JIT compiler and one nice area that improves is regex handling so all of my log-related work got faster with zero effort.