A couple of reasons.. You won't be measuring the heat generation by the fermentation, but more importantly the sensor will be too sensitive. As soon as it gets a bit of cold air on it, it will shut off, then it will turn on again shortly after that. Some of that can be prevented in the unit itself, but hard to say without buying it and testing that. Using the ebay controller I put it on the carboy under a sock and then used a bungee cord to hold it in place. The cycle time was pretty low, and held a pretty steady temperature.
Also, this is a single stage controller, you will only be able to cool, not heat and cool. Depending on your environment it may or may not be an issue.
I converted my freezer with the ebay controller and it works great! The whole project only took a few hours, which includes building a wood collar to increase the height. The hardest part is finding the right part on ebay. Make sure it is 110volt and it is a dual stage controller.