I think almost all musicians who learned both instrument would agree that violin is harder than the piano.
One of my old professor said that the only human endeavor more difficult and more delicate and complex, requires more practice and knowledge applied is a brain surgery. The other said that violin playing, at the high level, is 10000 times harder than what most people perceives.
I mean is it harder to be a piano virtuoso, or a amateur violinist? Perhaps the former. But with all things being equal, the violin is definitely the harder instrument to learn and play.
The reasons being that...
for the violin, you need to MAKE the sound instead of just pluggin/tapping on the keyboard to make notes that are already "crafted out" for you.
The fingerboard on the violin is so fricking small that it's like trying to fit a piano in there without lines or divisions between notes to dictate where one note start and the other ends so it's extremely to play in tune...especially the higher notes where 1 mm off in your fingering position and the note is off! It's amazing how we are able to know where all the notes lay on that small piece of wooden board.
Not to mention that you have to know how to control the right AND left hand simultaneously, under 2 completely different mechanics.
I think however, the hardest thing about the violin is the shifting of the positions. IMagine a computer keyboard. Every letter on there is set in stone, so to speak because typing the letter "A" would always require your hand to be in the same position each and every time.
Imagine that there are 10 - 12 keyboards in front of you, and just to type a sentence as short as this one, it requires you to switch back and forth between several different keyboards where every letter is placed in different configuration and spacing....not to mention that there are no visible "KEYS" on the violin fingerboard at all for you know to exactly where to press!
that is what it feels like to play the violin. For every position changed (which is required hundreds of times through out a piece of music because one position only allows your fingers to have enough space for you to play 2 octave, and it is not uncommon to play a passage that requires jumping between 4-6 octaves...one after the other as a arpeggio does. This means that you would NEED to "shift" aka finding a new keyboard (without notes or letters written in to indicate what note you are tapping) to complete to an entirely different set of fingerings and ways of playing.
Lastly, violin is one of those things you do that if you don't start out when you are 5 years old, and if you don't mold a child's brain along with the instrument when it's still developing, it's impossible to learn it later in life.
On the piano, guitar, and most other instruments you can think of, one can sound "alrite" if one starts out late. You can still play tunes to impress the ladies who don't know anything about the instrument or the music with 1-2 years of diligent practice.
But on the violin, it's hard to make a sound even to please an untrained ear if one starts out learning the instrument too late. Most adults that try to pick up the violin fail miserably and quit after 2 weeks.
I hope i did my best to explain how difficult it is. That's why i hated the instrument growing up, when forced to practice. I think it's child abuse to put your child through this. In a way I am glad that i did.