What is the primary weakness of sensitivity analysis? What is its primary usefulness?

MINI CASE 

Shrieves Casting Company is considering adding a new line to its product mix, and the capital budgeting analysis is being conducted by Sidney Johnson, a recently graduated MBA. The production line would be set up in unused space in Shrieves’ main plant. The machinery’s invoice price would be approximately $200,000; another $10,000 in shipping charges would be required; and it would cost an additional $30,000 to install the equipment. The machinery has an economic life of 4 years, and Shrieves has obtained a special tax ruling which places the equipment in the MACRS 3-year class. The machinery is expected to have a salvage value of $25,000 after 4 years of use. The new line would generate incremental sales of 1,250 units per year for four years at an incremental cost of $100 per unit in the first year, excluding depreciation. Each unit can be sold for $200 in the first year. The sales price and cost are expected to increase by 3% per year due to inflation. Further, to handle the new line, the firm’s net operating working capital would have to increase by an amount equal to 12% of sales revenues. The firm’s tax rate is 40 percent, and its overall weighted average cost of capital is 10 percent.