- They are hard to track.
- Changing all may require changing multiple files in possibly multiple projects (hard to maintain).
- Sometimes itsit's hard to tell whatswhat their purpose is just by looking at their value.
- No reuse.
Copy edited. [(its = possessive, it's = "it is" or "it has". See for example <http://www.wikihow.com/Use-Its-and-It%27s>.)]
