When I tried to call the get_activity function with for instance since set to 2014 and until to 2017, the returned dataframe contained only a part of the real data which should be returned for this time interval.
As a workaround, I wrote some code that divides a time interval in multiple one-year intervals, passes each of those to get_activity and at the end, the resulting dataframes are merged.
I do not think this is a limitation of the github-activity tool, but rather a limitation of the GitHub graphql infrastructure, but which unfortunately affects this tool too if a user tries to pull activity spanning over multiple years.
If you can reproduce this issue on your own setup and consider it is a good idea to have this integrated into this repo, let me know, I'd love to submit a PR.