I only know how to play guitar and drums, so I'll say guitar. I could teach somebody who has never picked up a guitar in their life to play a three chord progression "song" in 30 minutes.
Now, guitar (and I'm sure any instrument) can be as brutally hard as you want it to. Want to be able to sweep tap-pick 16th notes at 180 bpm? You'll have to practice that every single day for 6 months straight before you can do it, and even then it won't be clean.