What exactly are we talking about? A TC is supposed to lower IQ, even an optically perfect one, and the higher the magnification, the more it will do that. What it is supposed to improve is SQ, or "subject quality", not full image frame quality or the quality of pixel-level views. One can not expect to stand 2x as far from a subject with a 2x TC, and get the same IQ. That would be an expectation of magic, as you need 2x the shutter speed when 2x as far away, to have the same subject-level stability, and you have 2x the diffraction blur size and aberration blur size, relative to the subject size, with the lens wide open, and there is also about 2x as much atmospheric pollution between you and the subject. These are all results of being 2x as far away, not the results of using a TC! A TC's main practical purpose is to avoid undersampling of a subject that you get with heavy cropping, not to overcome the other issues with distance, through magic.
We have had cases in the past where a manufacturer's 2x TC had more contrast loss than their 1.4x, which is a true TC flaw, but modern OEM TCs tend to be very good these days. With DSLRs, TCs would often create small back-focus/front-focus offsets in AF, or fail to focus or refuse to even try because of the open f-ratio, but with sensor-based AF, these are much less frequently a problem, and the worst issue is usually just slower AF in poor light or low subject contrast. although sometimes a TC will make AI AF that identifies things like faces and eyes easier to see with smaller subjects.