The foremost limitation of anything magical is experience and training. Physical shapeshfting is harder than glamour but it's more reliable if you are a good actor and trying to impersonate someone. 

turning into different animals is the hardest of the bunch but not impossible. It takes time to shapeshift but that all depends on how experience one is in such magics. Usually turning back into the original form is quite easy compared to transforming into a dog or something else. Small creatures/creatures of similar size as you are usually the easier (Broad term) to do but the larger the creature is, the harder it is going to be. Again, compared to training. But it is continuous magic output is not needed for such magics. Once transformed, you don't need to maintain it. Just hope you don't get stuck that way.