I play several instruments and like others have said, each has things that are easy to do on them and things that are harder than you would expect.
That said I would put Harp fairly high up on the difficulty level.
As someone mentioned, one of the things about the harp that is an advantage is that you can produce a decent sound right off the bat. Certainly to get a really great sound takes practice, but unlike many other instruments you can make a nice sound your first time sitting at a harp just by running your fingers across the strings. You can’t do that on a flute.
The harp is easy to learn to play basic, simple yet nice sounding tunes. Particularly diatonic music. Once you get to more advanced arrangements or more chromatic pieces then the difficulty increases exponentially. When you are talking about chromatic music on a pedal harp you are now in the really difficult range of experience.
Yes, you can play in any key on your first day of playing, but if you are going to comp over chord changes it is much easier on a piano where you can visually see the relationships between the notes than on the harp where the strings may play one note one moment and a different note the next and you basically have to keep track in your head what note the string is going to play when you get to it.
On top of that the music is equal to piano music in terms of difficulty. Playing music which includes chords and melodies and counter melodies on a grand staff is just plain hard. As one music teacher told me once, people give up the flute or the violin because they can’t get it to sound good, not because they can’t read the music, but people give up piano all the time simply because the music is so hard to read. And that goes for harp too which uses the same basic kind of music and staff.
And of course the more advanced the music you are playing the harder it is to keep a good sound. Once you start playing fast, chromatic music filled out with lots of chords and such it gets harder to produce good tone, and you are more likely to buzz against another string.
I would put harp up there with pipe organ in terms of difficulty to play really well.
But, it is easy to start. In fact easier to start than most instruments.