The z5 has got a wider field of view and when doing comparison with other cameras with different fov, it places it in a "disadvantage ". When you crop the image produced by the z5 to compare,you are actually zooming it far further than the rest giving it a obviously more pixelated result. What you should do for the z5 is to move forward to your subject to frame it similar as the other camera, doing so "corrects" the fov for fair comparison.

Sadly i see many review sites and youtubers doing the same thing without correcting it giving the impression that the z5's camera is inferior which it isn't. Also reviewers like to use the z5 full 23 mp auto for their camera "battles" which is really not best setting for it. The z5 really shines when using its 8mp. I know it doesn't make sense how 8 can be better than 23 but that's when Sony's magic happen. It supersamples its full 23 mp sensor to 8mp giving it a clean low noise and detail rich image.
Coming from the z1 the camera is a stark improvement. I own both the z5 and s6 so i can post some real shoot out as well but I'm too lazy to do that...