Let me verify that I'm understanding correctly; these are "muscles" between bony plates that are honeycombed, and inflate themselves with fluid to push those bones apart, yes?
It could work, sure. In-between positions would be achieved the same way humans do it; by matching exertion against resistance. Flex a hydraulic 'muscle' a little against gravity, and that's halfway. Flex two hydraulic muscles against one another, and you would have a halfway position, too. Further, the specific inflation of each could create a 'dead zone' between the pressure exerted by each muscle, so that the position could be maintained without stress.
The problems I foresee are numerous, though. How much pressure can they exert before cells (of the honeycomb structure) begin to rupture? Once one cell ruptures under pressure, it would increase the pressure on other cells, leading to a cascade of failures. (Not that our muscle doesn't face similar challenges.) Hydration woulde be super-important. They could be incapacitated with a strike to whatever fluid reservoir holds the excess. They would have a limited capacity, which means perhaps not being able to use all their muscles at once, at full performance. The need to shift fluid back and forth means their response would likely be slow.
I could see this being a supplementary or 'idle mode' system. When going about your day, moving slowly without need for fast response, you can rely on these. When you need great strength, lifting and carrying or supporting a lot of weight, but you don't need to move fast or responsively, hydraulic muscles might serve you, assuming (as the links suggest) that they're capable of greater strength.
When you need to run or fight or throw a spear however, it doesn't look like they'd be capable of the kind of explosive power and speed that a fighter needs. They sound more like they're specialized for power and endurance.
