You've kinda hit upon the reason why there are few pneumatic robots.
There are no viable valves. Proportional servo valves are big,
expensive, and power hungry. Dynalloy
uses nitinol to
make a proportional valve. It's small, quiet, and less expensive than
comparable solenoid or servo valves ($8 when ordered in bulk), but it's
still difficult to drive and a huge power hog. Plus Dynalloy only offers
it as a demonstration product, so you have to buy tons of them for the
price per unit to become reasonable.
Maybe in a decade we'll see some improvements with polymers that'll
solve the problems inherent with SMAs. But for now robotic pneumatics is
a bleak and cumbersome field.