Multiple problems for any shape shifter:
Where is the additional mass coming from? It wouldn't be instant; they would have to eat and digest a whole lot as they grow whatever extra limbs. Even if they are physically capable of rearranging their physiology, any additional growths would take time, it would be awkward and inconvenient. Look at how inconvenient and time consuming pregnancies are. It would probably be painful to walk around with a half-grown limb too.
If the shapeshifter tries to make themselves bigger, they'll rapidly hit the cube square law. Bones will be strained, joints will wear. Even real life tall people can easily have bone problems. The shapeshifter will have to take steps to repair any strained musculoskeletal aspects.
Even if you can grow an extra arm, that doesn't mean you can control it. Motor control comes from the brain, and the nerves. You can't just copy and paste - you'd have to edit your own brain patterns to make use of it. And anything editing brains is risky business.
Which might be the biggest problem - this shapeshifter (presumably) isn't omniscient concerning biology. In order to redesign his own body, it has to understand how it's body works. You'd need a medical degree just to get started. Even the best doctor in the world wouldn't be able to rebuild a complex organ piecemeal, cell by cell.
Biology is delicate. Changing it is dangerous What happens if the shapeshifter makes a mistake? There's a reason most engineers try to deactivate a machine before modifying it, but the shapeshifter can't do that. He's running live, constantly; he has to make changes as he lives. What if he accidentally changes his chemistry too much or removes a vital bit of nerves as he shifts, and ends up catatonic or dead? He's got no safety or reassurance; his entire existence is in his own hands.
So overall, even if the shapeshifters could change their own biology to an unlimited degree, I don't think they'd be eager to try. A shapeshifter would maybe have one or two established and well-tested forms that they vary between as required - but trying to shapeshift freestyle would just be too difficult and risky.
Maybe a community of shapeshifters would get together and try to figure out the best shape. Still, i reckon these shapeshifters would have more medical issues than any other race combined; they are effectively amateurs that have been given admin access to a complex machine. There is no end of ways that they could mess things up while fiddling around.
All things considered, it might be easier for a shapeshifter to just... not use their powers. Live as a regular human, it's not so bad.
Realistically, what are these shapeshifters going to use their powers for? Humans are already pretty well designed for all activities humans do. Growing an extra limb or wings seems like a lot of trouble for not much gain, really. Unless they have a very specific application in mind, it'd be easiet just to stick as a human.
The most useful part of shapeshifting would be to strengthen and reinforce, rather than change. They'd be biologically immortal, if they could constantly update their cells to avoid aging.