Core hacks are generally not advised, unless they are official hacks released by the xoops development team and intended for integration into future official xoops realeses.
Core hacks developed by any other third parties will not necessarily be considered for inclusion by xoops future releases, so any core changes will not be accounted for.
This can lead to problems in updating your files. Any official patch or new release may overwrite some or all of the third party core file hacks and remove/corrupt its functionality. Worse still, some files from the unofficial core hack may interfere with new updated core files and corrupt your system from working.
Your solution is to receive a further updated core hack file from the third part developer to be compatible with all version updates.
This relies heavily on third party developers maintaining their support.
In future QA classification of Xoops modules, any required core hacks will be regarded as a lower standard of module construction.