The only problem I have with air muscles is that they dont seem quite so reliable yet. Ive seen a few little demos of some and the muscles appear to steal a significant amount of air from eachother if your moving more than one at a time. It looks like you would need a lot of pressure in all your lines to account for that...
Only because the mechanical engineer on that project improperly spec'ed out the required pressure in the system
So if you have two muscles, each requiring 10 units of pressure, that means your compressor needs to deal out 20 units. If the muscles leak 2 units each, thats 24 units of pressure needed. If your compressor supplies less than that, then yea, the muscles will be much weaker when both are on.
But there could have been other reasons for the under spec'ing, such as financial reasons (bigger compressors are more expensive), or the system was not fully understood during the design phase (which happens often in research situations).