It would be great if users had an option to auto-assign tasks for the duration of the project. Ideally, this would be an option after '6 weeks' in the schedule when only one project is showing that would dynamically set the number of weeks to match the project planned start and end dates. However, if it's a tick box on auto-assign that means I can't see the entire project and I have to scroll, so be it.
Yes, if you have a project that spans years it might cause problems. I'd suggest you don't select that option or put it back to 6 weeks in that case.