The almighty teleconverter lens

Let's me explain what is teleconverter lens? And what does it do?
Basically, a teleconverter (also called a tele-extender, converter or extender) is an optical device that fits between your camera and the lens.
The main function is to extend the telephoto focal lengths without buying a totally new telephoto lens. It has a few magnification factors typically ranging from 1.4x to 3x. So if you fit a 1.4x teleconverter lens between your camera and a 300mm lens, you will get 420mm (672mm on 1.6x crop camera!) instead. Simple as that! Now you can have 2 focal lengths lens with minimal cost. Typically, it costs ranging from SGD190 to SGD600, depending on the magnification factor and brand name. As for me, I got it off from ebay which cost me about SGD190 for a Kenko 1.4x teleconverter lens. And its optical quality is almost as good as Canon 1.4X which costs about 600 bucks! In addtion, the Canon one only compatible with Canon lenses whereas Kenko can work well with other brands such as Sigma and Tamron.
After listening all the good things about this lens and now let's find out what are the drawbacks.