The typical complaint of designers and clients is that they have spent much time and often very much money, designing the perfect space for the clients' needs and then - the contractor comes in, views the plans as suggestions, doesn't order the custom tile until the day of installation so - oops - we need to reselect the tile, etc You get the drift. The result is a space that has nothing in common with the space that was so meticulously designed. This happens more than you think.
The other situation I run in to is a lack of communication and/or organization that results in emergency decisions. One in five people that call me for design services start off the conversation "My contractor says I have to pick the paint color (tile, lighting, carpet, etc) TODAY or the whole project has to stop and it will cost me more money."
When you hire a designer that is also licensed as a general contractor, there is no question what the lead time is of the product that was selected. The designer/contractor is experienced at coordinating purchases and installations of custom materials and finishes. When you hire your designer as your contractor, there is not a break in the design focus. The project will be constructed the way you and your designer envisioned it. That is why design/build companies are so popular.
One word of caution - make sure the designer (of the design/build company) is one that you would have hired as your designer. You have a choice to have either a 'nice' remodel or a 'WOW this is fantastic!' remodel. The difference is the Designer.