Zacian and Zamazenta aren't exactly hard, more so tedious. You have to beat the entire game, and finish a large amount of the post-game to get your opportunity to catch them.
Stonjourner/Eiscue, if not using the DLC, have just two places they can appear, they're both late-game areas, and extremely rare in both.
Dreepy can only be found in one location, with roughly a 2% chance of being found in a specific weather condition.
Dhelmise has a 1% spawn rate on the route outside Spikemuth, making it very hard to obtain. This can be boosted with Harvest, however.
Authentic Sinistea can only be found in the Glimwood tangle, and it's even rarer than fake Sinistea.
Rotom is in pretty much the exact same situation as Dreepy.
Feebas can only be found on one route (Route 2), at a 1% encounter rate.
A shiny antique Sinistea is quite possibly the rarest Pokemon in the entire game.
This answer does not take into account Max Raid battles.
Hope I helped!