When unlocking and moving into the new nations, we can encounter new mobs, and along with that, the difficulty of the area would increase slightly as well. Primarily because there is a certain adaptation period that comes with the new monsters, Inazuma was definitely an obstacle with the introduction of the Nobushi, Specters, Rifthounds, and Primordial Bathysmal Vishaps. However, we have only Eremites, Ruin Drake; Earthguard, Ruin Drake; Skywatch, and all sorts of Fungal monsters in Sumeru. These new monsters are not necessarily a step up in the difficulty scale, but players can get overwhelmed by the large number of monsters that can spawn. I believe that Sumeru has a little more to offer as the newer versions come out; we can only wait and see what happens.