Some versions of shape shifting have very real downsides. Remember Odo from Deep Space 9

Notice how his face looks like it was crudely sculpted out of clay? This is because in the story he's a shapeshifter. But this isn't shapeshifting like Animorphs, where you just have to imagine an image of an animal that you've absorbed the pattern of. That form of shapeshifting is like a menu. You mentally select a form to shift into, and the powers take care of all the pesky details.

But some forms of shapeshifting, like Odo's, don't work like that. You can't just mentally tell your powers "make me look like John." If you want to look like John, you instead would have to mentally picture every single minute detail of John's face and body. Imagine if you had to consciously recall and build up your entire form detail by detail every time you shifted.

This is why Odo looks so odd. His true form is a puddle. Him taking a form is like a painter painting a picture - every detail has to be consciously chosen. Maybe a true master painter could shape shift into a form with a realistic face, but most folks would end up looking like Odo.