Slide 42 of 89
In their 1997 book, Computer Architecture, Blaauw and Brooks criticize the 6600 as fundamentally being ad hoc. In nearly all of the Cray designs, there is an assumption that it can be programmed to exploit the inherent parallelism of a functional design.
The criticisms such as the specialized registers though valid, fail to acknowledge the implications of a single register set would have on implementation and the difficulty of control with a more general register structure. Because there were 3 different types of registers, allowed for functional parallelism and simpler control and more limited fanin/fanout than with a larger set of registers.