Well I use both types of tele-converters depending on the camera I am using.
I use a 2x auxillary lens, and also a fisheye and wide auxillary lenses on my Nikon CoolPix 950 and have had no problems at all. All three have had shots accepted here on A.net.
I also use a 2x on my SLR Nikon and again no problem. However there is a two-stop loss of light with the converter in place. It should be noted that if your prime lens is only a f:4.5 or so or a slow variable aperature lens, then the quality will suffer somewhat and the autofocus will most likely have problems due to the lower light levels reaching the AF
sensors. My prime telephotos are all f:2.8 lenses so they work well, including autofocus with the extenders. This includes the 80-200mm f:2.8 and the 300mm f:2.8 lenses.
Perhaps why the quality stays good is that they are all Nikon converters and lenses. (no spam please)