We build a code review product that interfaces with over a dozen SCMs. In about 20 years of writing diff parsers, we've encountered all kinds of problems and limitations in SCM-generated diff files (which we have to process) that we wouldn't ever have expected to even consider thinking about before. This all comes from the pain points and lessons learned in that work, and has been a huge help in solving these for us.

These aren't problems end users should hopefully ever need to worry about, but they're problems that tools need to worry about and work around. Especially for SCMs that don't have a diff format of their own, have one that is missing data (in some, not all changes can be represented, e.g. deleted files), or don't include enough information for another tool to identify the file in a repository.