Whilst transfiguration was an exceedingly useful branch of magic, it naturally had its own set of limitations. 

usually, a wizard or witch attempting to perform a transfiguration spell would often find it easier to transfigure one thing into something else provided that the state it was originally in had something in common to what it would be converted into (i.e a statue of a dragon turning into an actual dragon, albeit the same size as it was when it was a statue). The overall mass and size of the object or entity to be transfigured into something else must also be considered, since it was stated by Hermione Granger, that it extremely difficult to transfigure something as large as a dragon, even by a skilled transfiguration expert like McGonagall. Furthermore, transformation was limited by the fact that 

the dead could not be revived via magic and hence one could not ‘’ transfigure’’ the dead back to life, no matter how recently the death occurred. Conjuration was principally limited by the five principal Expectations to Gamp’s Law of Elemental Transfiguration. The first (and only known) of these was the restriction that one could not conjure food (though, pre-existing food could be charmed in certain ways — ways such as by summoning and multiplication — and transfigured by the other branches). Similarly, Wizarding law set legal limitations on what one was and wasn’t allowed to conjure. Finally , even if it was both possible and legal to conjure something , it would ultimately never last.