effective aperture and light transmission becomes less, potentially requiring higher ISO or slower shutter speeds
degradation of image quality
may require stopping the aperture down 1 stop to attain sharpness if it is a high quality teleconverter
lower quality teleconverters can give poor image quality no matter what the aperture
if one needs to increase ISO to compensate for the aperture loss, then this will add to image degradation
may affect quality of bokeh and degree of distortion, coma, astigmatism, spherical aberration and CA
TCs will also lower contrast by adding internal reflections and flare
image quality issues are more likely to be evident when used with zoom lenses rather than prime lenses
may affect AF accuracy
generally result in slower AF due to less light getting to the sensors and failure of PDAF capability
most dSLRs have cross-type AF sensor points which revert to non-cross at apertures of f/5.6 or smaller
in fact, the decrease in aperture may make AF not possible in some cameras (especially dSLRs in which the AF points often do not function if the effective wide open aperture becomes less than f/8)
may alter the biomechanical ergonomics of larger lenses by pushing them further from the camera
may affect weathersealing
additional electronic lens communication pins means increased chance of failure of lens communication, especially if the pins are getting tarnished or misaligned
additional mechanical device means increased risk of optical misalignment and wobble of the lens
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they may be designed ONLY for certain lenses
they may NOT be designed to be stacked - some do allow a 1.4x to be used WITH a 2x to give 2.8x effect but most modern ones do not