"pt-online-schema-change modifies data and structures. You should be careful with it, and test it before using it in production. You should also ensure that you have recoverable backups before using this tool."
With that being the case what im doing is fine I guess.
"INSERT modifies data and structures. You should be careful with it, and test it before using it in production. You should also ensure that you have recoverable backups before using this tool."
"ALTER modifies data and structures. You should be careful with it, and test it before using it in production. You should also ensure that you have recoverable backups before using this tool."
It's just good advice to test stuff before you use it in production. Discarding an awesome product simply because the authors (responsibly) mention that you should probably test it out seems overly paranoid and would severely limit your choices. Percona Toolkit is the most well known and reliable set of tools out there in the MySQL ecosystem.
Maybe they changed it in later versions of MySQL, but adding a column to a table become so lengthy for some of our projects that we switched for that reason alone.