You don't want to replace a slash with an underscore. Google views slashes as word separators but underscores are parts of words. See http://www.mattcutts.com/blog/dashes-vs-underscores/
When slashes represent physical directories on the webserver, many slashes in a URL indicate that the file is stored many folders deep on the server. This often means that there are more places where a reorganization can cause the URL to change. It also often means that more people are involved in keeping the page live. For example
/users/~jphillips/friends/adam/homepage.html needs cooperation from Adam, J Phillips, and the server owner to keep it live.
There were some studies years ago that showed that the number of slashes in the URL was correlated with how stable the URL was likely to be. More slashes meant that the URL was likely to change or go 404 sooner.
I have never seen any direct evidence that Google started using the number of slashes as a ranking factor. Googlebot can detect URL changes and deletions. Then it updates the search index accordingly when changes occur. Because of this, Google shouldn't need to rely on how likely a page is to change as part of their ranking signals.
Focus on creating URLs that are:
- As short as possible (while still being descriptive)
- Use word separators such as slash or dash but not underscore.
It isn't worth worrying about minor differences in punctuation in the URL for SEO purposes.