This may be a dumb question, but can pieces such as Tzigane or Cello Concerto in B. Minor, OP.104, be played on the piano?
The notes tend to be accessible. However, the piano is a percussive instrument (though with less "off-topic content" in its attack compared to, say, the quill noise of a harpsichord) and bowed string instruments are continuous tone instruments. That means that you just can't be similarly "in-character" as, say, a bass accordion rendition of the prelude from the Bach cello suite #1 because you are lacking the necessary continuous control over the tone.
Indeed, you'll likely even run into some range problems with something like the fugues from the Bach violin solo Sonatas or the Ciaccone from the second solo Suite: that's because they are polyphonic, and while the total range of a violin is much smaller than that of a piano, the simultaneously accessible range on four strings (discounting that you'll need to arpeggiate on modern instruments) may actually be tricky to cover well with two hands when open strings enter into the equation.
There are also some nicely voiced polyphonic passages making use of polyphony distributed across strings with voices containing equal notes and/or crossing. That makes even something formally monophonic like the prelude from the violin solo partita #3 hard to render adequately on a piano.
In many cases, you'll still be able to produce the notes well enough to act as a prompter for a violin (like the piano is often employed in support of aspiring singers), but sometimes the natural magic will be gone and will require a lot of skill to artificially reinstantiate to some degree.