I worked on a code base which had a large dependency not port to Python 3 until 2018, and at the same time they made a huge ton of changes to the APIs, most of

I worked on a code base which had a large dependency not port to Python 3 until 2018, and at the same time they made a huge ton of changes to the APIs, most of which were not documented well. So there was both a big refactoring to handle binary data transfer between it and the application, and also a ton of other unrelated things just to make it work. It's things like that that made it difficult to port for most people.