Limited Materials
3D printed parts are built in additive fashion -- that is, layer-by-layer from the ground up. While the technology is a major process breakthrough, the materials that can be used are still limited. For instance, the 3D printing material of choice is plastic, as it can be deposited down in melted layers to form the final part. The kinds of plastic vary among the likes of high-strength and high temperature materials, so part strength can't accurately be tested in many cases. Some developers are offering metal as a material, but final parts often are not fully dense. There are several more specialized materials that companies are printing in, such as glass and gold, but such technologies have yet to be commercialized.
Questionable Accuracy
3D printing is primarily a prototyping technology, meaning that parts created via the technology are mainly test parts. As with any viable test part, the dimensions have to be precise in order for engineers to get an accurate read on whether or not a part is feasible. While 3D printers have made advances in accuracy in recent years, many of the plastic materials still come with an accuracy disclaimer. For instance, many materials print to either +/- 0.1 mm in accuracy, meaning there is room for error.
Manufacturing Limitations
3D printing is perfect for creating prototype parts because it's an economical, inexpensive way of creating one-run parts for which you don't have to create tooling. Parts typically are created in hours and changes to the design and engineering of the part can be made in a CAD (computer-aided design) file after the part is analyzed. But in terms of a manufacturing process, 3D printing is not a realistic option as of the date of publication. In manufacturing processes such as thermoforming and stamping, several parts are typically made in one minute, not hours.
Size
Parts created additively through 3D printing are also limited in size. For instance, the most affordable, common 3D printing machines typically are small enough to fit on your desktop, meaning they have build chamber sizes of similar proportions. There are 3D printers that are able to create larger parts, but they're much more expensive and thereby an unrealistic option for many companies. As a general rule of thumb, the larger the part that needs to be fabricated, the longer it takes to create