It sometimes provides an incomplete result when solving non-linear equations. For instance, doing

   solve(Eq(w*x + y*z,0), [w,x,y,z], dict=True)

returns the incorrect result

   ⎡⎧   -y⋅z ⎫⎤
   ⎢⎨w: ─────⎬⎥
   ⎣⎩     x  ⎭⎦

which is invalid when x is zero. There are more compelling examples, but I don't know any off the top of my head. Interestingly, Wolfram Alpha seems to be better at returning the whole solution, so I use it in combination with Sympy.