I just find single variable much easier and simpler. Maybe it's just me, but everybody in my class felt the same way, so that's gotta say something.
Ah yes, spherical and cylindrical coordinates. Those help with the tricky geometries, but I'm talking about **** like tetrahedrons. One of the most basic 3-D shapes, and I still had a hell of a time figuring out the bounds. Maybe I just didn't get it; that's entirely possible, but I remember my professor, during the final exam, had us find the bounds of a tetrahedron, and then graded it right there on the spot and gave us the correct bounds to solve the integration, because he knew how much trouble we'd have and that we'd be ****ed if we tried to do the integration with incorrect bounds. Smart move, I remember messing up the bounds, hurrr.
So many bad memories! I could never be a math major. Props, bro.