>>96139207 (OP)Stable portals, by definition, do not destabilise their local environments.
If you mean a persistent portal with no pretensions of stability, then that could be a problem for either side.
For the sake of argument, perhaps you mean socially or technologically disruptive by the term "world-breaking." In which case perhaps the other world is boring? Imagine a medieval world who get access to a stable portal to a moon-like environment. Airless, filled with (to them) unremarkable rocks, etc. It would be a remarkable phenomenon to witness but fundamentally no more important than the Bolton Strid.