There's a level of quality and time creators need to put into the meshes they create. I know, I make mesh templates. For a creator of a kit to say it's your fault.. well it seems appalling to me. I myself have bought meshes that I've been extremely disappointed in, so much so that I decided to make my own. So I know the feeling Tarina.
Definitely buy the demo. Always. Any creator that doesn't provide a demo for mesh clothing, in my experience, shouldn't be frequented. As each creator makes the meshes, whether or not they use the standard sizing, there are always differences.. mostly minute, but sometimes dramatic. Always check the demo.
As for it not fitting properly on the waist/bust/hips/bottom, there is very little you can do about such things at this time, however that will -hopefully- change when the mesh deformer finds it's way to the general public. There are workarounds right now, though. I, for example, offer custom sizing to avatars, and have done complete sets for stores who chose to cater to their customer bases rather than trying to force them to conform to the standard sizes.
What you should expect from full permission mesh clothing is this Tarina: A reasonably well made product that moves well. Even if it requires the use of alpha layers (which they do at times, especially at joints such as the elbow, knee, and hip), the alpha layer shouldn't be noticeable. Problem areas (the shoulder specifically) tend to be harder to deal with. If it doesn't fit, or eats into your body more than a little (with the use of an alpha layer), then it is simply not worth purchasing.