Con--Client may select an older style or theme from your portfolio that you have grown out of.

Pro--Intellectual challenge of designing with constraints.

Con--Client may request colors you don't normally use outside your comfort zone.

Pro--Guaranteed sale.

Con--Possible that you won't want to include it in your portfolio because it "sticks out" or you feel like too much of your aesthetic was compromised to make it.

Pro--When the client gets exactly what they want and are happy.