I too thought a lot about it in this beginig and already there is a lot of discussion in this forum.

But you know according to PMBOK, you can put a "NOT to exceed amount for the T&M amount" but this option is not there for CPPC or CPFF.

This is the statement from PMBOK

"These types of contracts resemble cost-reimbursable contracts in that they can be left open

ended and may be subject to a cost increase for the buyer. The full value of the agreement

and the exact quantity of items to be delivered may not be defined by the buyer at the time

of the contract award. Thus, T&M contracts can increase in contract value as if they were

cost-reimbursable contracts. Many organizations require not-to-exceed values and time limits

placed in all T&M contracts to prevent unlimited cost growth."

Now lets see the definition of Cost Reimbursable contract

This category of contract involves payments (cost

reimbursements) to the seller for all legitimate actual costs incurred for completed work, plus a

fee representing seller profit. Cost-reimbursable contracts may also include financial incentive

clauses whenever the seller exceeds, or falls below, defined objectives such as costs, schedule, or

technical performance targets. Three of the more common types of cost-reimbursable contracts

in use are Cost Plus Fixed Fee (CPFF), Cost Plus Incentive Fee (CPIF), and Cost Plus Award Fee (CPAF).

A cost-reimbursable contract gives the project flexibility to redirect a seller whenever the scope

of work cannot be precisely defined at the start and needs to be altered, or when high risks may

exist in the effort.

As you can see here, in comparison risk is only mentioned for Cost Reimbursable contract wheras Time and Material contract may or may not increase plus you can define a not to exceed amount.