SMT: SJ really isn't that hard. The issue is that the difficulty curve is all over the place. Some of the BOSSES are excessively hard compared to the regular fights, which are all generally pushovers. The bosses aren't really hard in and of themselves, I guess, just... comparatively.
In SMT:N the regular fights were all potentially lethal and the boss fights were still a lot hard but it wasn't such a wild oscillation.
Also the frustrating elements in SJ are more the goddamn invisible doors.