Unicorn hair generally produces the most consistent magic, and is least subject to fluctuations and blockages. Wands with unicorn cores are generally the most difficult to turn to the Dark Arts. They are the most faithful of all wands, and usually remain strongly attached to their first owner, irrespective of whether he or she was an accomplished witch or wizard.
Minor disadvantages of unicorn hair are that they do not make the most powerful wands (although the wand wood may compensate) and that they are prone to melancholy if seriously mishandled, meaning that the hair may ‘die’ and need replacing.
(Source: remusjohnslupin, via crookkshanks)