You can take any study and find fault.  As has been pointed out If the CPP1/2  doesn't do well on returns then most likely your personal investments are in the same boat.  A couple of years go an actuarial study of CPP1 declared the plan sound for the next 75 years (the max period they can say so).  According to the CPP website I believe they stated their return was, i believe, 5.8% over the previous 10 years, including our recession period.  
It is not the politicians that calculate the plan financing but actuaries and mathematicians who are trained for such and have standards that they must adhere to.  It is standard to have lower return rate than current market.  Any plan forecasting has the same risks, cannot be avoided.  
The CPP management do not invest only  in the stock market, like we might, but buy into infrastructure.  CPP owns airports ,bridges, highways, buildings, docks, and only after months of study.