Heat loss due to resistive heating of the thermoelectric elements is a factor.

I do some work using TECs for cooling optical sensors. Most of the industry has settled on a very small range of materials, such as BiTe, meaning that we're probably at a plateau until a more efficient material comes along. Some of my applications are battery powered, and cooling consumes a fair portion of the total power budget, so a more efficient cooler would be welcome.

Also, TECs are mechanically delicate. There's a laundry list of needs such as being able to form the material into the desired shapes, and make a good mechanical and electrical connection to it, which is usually done with some kind of low melting solder.