>>40911051.4x are usually solid, and you'll see a slight drop in sharpness with most 2x. Both are still better IQ than just cropping in most of the time.
If you have a mediocre lens, the TC can make the lens shortcomings (aberrations, etc) more apparent.
If you have a kit zoom like f5.6, you may lose autofocus depending on the camera, since you're effectively shooting at f8-11 instead.
