Why Are Arms So Hard?

Instructions unclear, added an extra joint

Arms aren't hard. Anatomy and characters in general are fun. Proper backgrounds are what's hard. Even most of the Renaissance masters cheaped out pretty hard in that department.

Use triangles instead of cylinders. What's being done in this video is incredibly dependent upon what the arms should already look like when foreshortened. Triangles will get you there organically.

But what if after you place hands you realize they can't be in this position and arms can't be attached correctly?

For me, sometimes the arm is just in an awkward area so even if you do block it out right, its more so the posing just looks awkward

Well, arms have more little bumps and stuff compared to legs, and more movement patterns where the internal muscle structure changes, so I also find, arms, well drawn arms, to be harder than legs. add foreshortening on top of that, and the difficulty spikes.