Space and heat are the limiting factors I see here, but the most heat and space efficient systems produce noise and don't look very human.
Space is less of an issue for larger muscles, but areas that require a wise range of motion with a lot of dexterity will be a severe issue. The hand, for example, has over 30 different muscles. About half are used to contract the fingers and thumb, about half are to contract the fingers and thumb, with the rest being wrist control. These muscles range from a few centimetres to several inches in length, with many muscles controlling hand movements located in the forearm and connected to tendons which then connect to the hand. How are we supposed to fit muscles, tubing, and coolant veins into a feminine and natural looking arm?
Pneumatic and hydraulic actuators are doable, but they'd have to be very small but still fairly powerful. Getting the force required for not only a quick but strong enough contraction will be difficult given the space restraints. I'm not entirely sure how we could fit everything into an arm without sacrificing too much strength.
Synthetic fiber coils take up far less space, but the downside is that they produce heat. Pneumatic and hydraulic systems also produce heat, but fiber coils primarily function by way of heat. When the coil has an electric current flowing through it, it will heat up and contract. When the current is broken it will extend as it cools down. Small singular fibers aren't so much of an issue, but when you start coiling those coils it becomes much more difficult to distribute that heat effectively, and may end up heating up other muscles that weren't supposed to be contracted.
This doesn't even touch on price price and availability, where synthetic fiber coils may end up being more efficient in the event that the temperature can be managed they aren't exactly being sold at the store. They can be manufactured, but the process is time consuming and complicated, plus isn't as effective as synthetic fiber coils developed by labs with better access to supply equipment.