As with all zoom lenses trade offs occur. First is materials, but it also keeps weight down. But at over half the price of 24-70 2.8 it is a more than adequate model. Pixel peepers may find complaints in corners at far zoom ranges. But there is no perfect zoom lens by any manufacturer even Leica.
But if you are a perfectionist stay away from any zoom and stick to primes. Depending on how you shoot I would recommend a fast 50, a tele like 180 or 300.