I just stumbled into the documentation for the package, which scores PCG64 (the 128-bit version) as "difficult to predict". This has always been an unsubstantiated, but also not refuted claim. You can now find here a simple C++ program predicting easily the output of a PCG64 generator from its output:

http://prng.di.unimi.it/pcg.php#claims

(BTW, thanks for implementing xoshiro256**. It turned out, however, that xoshiro256++ bulk generation vectorizes immensely better on AVX2 architectures, so this is what we are suggesting to use now.)