Simple rules can give surprising, complex behaviour. That's a good point. Unfortunately, it's also true not only of PEGs and recursive-descent parsing, but als

Simple rules can give surprising, complex behaviour. That's a good point. Unfortunately, it's also true not only of PEGs and recursive-descent parsing, but also of essentially every parsing formalism out there. So it doesn't seem to disqualify PEGs in particular. (It's also true of programming languages generally: For which inputs does Collatz halt, and why?)