The clamping system theory seems to depend on who you speak to! Stringway for example are big proponants of flying clamps and the JayCee stringing method uses them almost exclusively (please put me right if this is wrong). The theory is that in a flying clamp each of the strings is pulling in a different direction but with equal force, therefore assuming the clamp is properly adjusted and no string is slipping through the equal force balance each other out and therefore the drawback is zero. If it's badly set then the string may slip in one direction or the other and some tension will be lost.
However, the same is true of a badly set fixed clamp so both can be fine in the hands of an operator who knows how to use them correctly. I would guess (only speculating

) that it is easier to produce an decent string-job on a high end machine for a poor stringer than for a cheap machine.
The long story short...it's the stringer not the machine that counts!
Ash