WHY PEDAL POWER? Because pedal power utilizes the largest muscle group in the body, and a properly designed propeller is more efficient than oars, you can expect to go faster with more endurance than oar power. In addition, hands free operation opens up possibilities such as fishing or photography while under way.
WHAT ARE THE DISADVANTAGES? You won't be able to explore some places that you could with a conventional kayak. An oar powered kayak can operate in extreme shallow depths; often just a few inches. The prop drive of a pedal powered kayak hangs below the hull by about 12 inches, so you will need at least that much water depth plus a little more to operate. Another limitation of that propeller hanging down is that it can get fouled with weeds. The removable drive unit in my designs helps offset these disadvantages. By simply pulling the drive up, it can be cleaned if it gets fouled. It can also be raised up if it bottoms out (I always carry an oar just in case).