Photographs are not vector.
This means that logos that contain photographs will not always produce the same results with every application. Sure, a logo that utilizes a photograph can look phenomenal on a business card and really help to captivate an audience when uploaded to a website. However, when you re-size it and place it on something other than paper (perhaps on the side of a building or on sheet metal), you probably won't get the same stellar results.
You want a logo that can easily be scaled up or down if needed without compromising the quality of the design, and logos that contain photographs can present some major challenges when making these modifications. Make sure your logo is 100% vector.
Photographs are not timeless.
Avoid the temptation to use a photograph in your logo design. Photographs have a tendency to date your brand to specified period in time, which can be evidenced by the style of clothing that people are wearing, prevalent colors in the photo, hairstyles, and more.
So what really makes logo design timeless? Typically, the more neutral your logo is, the more timeless it will be. This usually means a symmetrical design with clean lines and a modernist structure. These are not qualities that a photograph can offer.
Photographs are not brandable.
A brandable logo design catches the eye and establishes a positive lasting impression of your business with your audience. The best logo designs are simple, scalable, memorable, versatile, and relevant. These are the characteristics that make your logo design brandable.
When photographs are used in logos, they negate the qualities that make for a good, brandable logo design. Photographs can make it more difficult for your audience to identify with your brand and run the risk of making your logo too generic. This could possibly cause your audience to confuse your brand for being representative of something else.
Your logo design could easily be the single most important aspect of your marketing. Rather than go about it alone and make a rookie mistake like using a photograph in your design, put your best foot forward and try hiring a professional graphic designer to create a logo that will help you stand apart from your competition.