I think playing a note on a guitar is not difficult because you only need to press a string on a fret to produce a note. It is harder to name or locate a note in the guitar than a piano. But what is hardest in the guitar is memorizing and learning the chords especially the hardest ones.
For beginners, the hardest chord to learn and to master is the F chord. Your index finger must be covering the whole first fret, your middle finger on the A string, 2nd fret, ring finger on the C string, 3rd fret, and your pinky finger on the F string, 3rd fret.
All your fingers will be used in this chord and positioned in a not-so-comfortable manner. Some regular guitar players find this chord still difficult to play especially in fast songs.