Choice of delta-neighbors in Balloon Hash algorithm
If you use the Mersenne twister or any other non-cryptographic PRNG, then it is conceivable that an adversary could exploit some pattern in the Mersenne twister's outputs to find a shortcut to reversing the resulting hash.  Cryptographic PRNGs are designed so that despite decades of efforts, cryptanalysts have been unable to find any patterns in their outputs that are even detectable, let alone exploitable in context.

Don't use the Mersenne twister!  It's also bad as a non-cryptographic PRNG because it is expensive to initialize and requires a multi-kilobyte state, so it discourages parallelism and reproducible subtrees of computations.