Most racks are not one-size-fits-all. They inevitably require some manipulating to make them fit. This seems all the more problematic with anything but wire racks.

Wire frame racks will allow you to bend them where you need them. Unless I was setting up a bike for randonneuring, or heavy hauling, that is the kind of rack I would lean towards.

The hefty, rock-crusher racks seen so much today are nice and strong. Tres useful when loads get really big - and they impress the heck outta other cyclitsts.

But for a lot of towning uses, they aren't really necessary. You gotta balance your needs.