The answer may be in the code of some applications.
What applications support Path Finder?
The vast majority of applications support Path Finder with absolutely no compatibility >problems, but some applications go one step further and had special integration built >in.
A small list of examples include:
Default Folder X
Some applications do require Apple's Finder to work properly as they are hard-wired >this way. If you would like to see compatibility added for one of your favorite apps, >visit our forum and let us know!
Their forum is located at http://www.cocoatech.com/forum/. See if you can find any applications that you use that are listed there that have issues. Finder is such an integral part of the operating system that replacing it requires applications not to call Finder or for Path Finder to intercept those calls I assume.