With advantages come disadvantages. The biggest disadvantage of using generic project management templates is that it limits the creativity of the user. The choices are limited and so is the look of the overall product!
Most generic templates are not unique. Several people may have found it on the Internet and used it ahead of you. It's also true that in many cases the colors or the layout cannot be altered or edited. For a unique template design, you might have to spend more money than you would like.
Limited creativity could also result in boring presentations. Using the same layouts and designs for the templates could bore the audience. Most would feel they have already seen your presentation, even though the content is new. Your target audience might ignore the new document completely.
Many times, generic templates put limitations on the text input that needs to be made. For example, in a template where there is space for 500 words and the user inserts 700 words, the template layout may change. It will not look good when it is stretched.
Templates work great for static pages. If this template is for a website and the website needs to be changed or edited regularly, one needs to build a simple template.
In some cases, templates eat up lot of space on the server. In addition, they might infringe on copyright issues. Finally, many of these templates come with their source file unknown. Users do not have access to the source files, putting the end-user at a major risk or reliability.