The issue as I understand it is that the comma is considered "reserved character" because it is treated differently when it is escaped in the uri by a user agent. This is an issue in the query string portion of the URI, and I do see commas used in successfully indexed URIs from time to time.

I researched this a bit recently for a client, and we decided to substitute a dash wherever the initial database grab comes up with a comma for a file or directory name. I don't claim my stance is definitive, but I do know it's safe.

2.2. Reserved Characters

Many URI include components consisting of or delimited by, certain
special characters. These characters are called "reserved", since
their usage within the URI component is limited to their reserved
purpose. If the data for a URI component would conflict with the
reserved purpose, then the conflicting data must be escaped before
forming the URI.

reserved = ";" ¦ "/" ¦ "?" ¦ ":" ¦ "@" ¦ "&" ¦ "=" ¦ "+" ¦ "$" ¦ ","

The "reserved" syntax class above refers to those characters that are
allowed within a URI, but which may not be allowed within a
particular component of the generic URI syntax; they are used as
delimiters of the components described in Section 3.

[ietf.org...] for the technical doc