You'd think the single spine would be the problem, but my experience of broken office chairs suggests that bit's usually properly engineered (I did have one where the gas lift got sticky, so you had to pull up to raise it, but that was only a problem if you wanted to adjust it). The weak points seem to be the plastic parts and anything where repeated flexing wears away the metal until it develops play, which then gets exacerbated by over-tightening things to stop the wobble. Typically they seem to get condemned because a wheel falls off, the back won't stay at the right height, or the arm-rests become detached. Sometimes the fabric gets torn, or the fasteners come out of whatever substrate the seat pan is built around (I suspect particleboard is often involved).
As usual, you do seem to get what you pay for, with office chair quality being roughly equivalent to that of a bike of similar price. The cheap ones simply aren't rated for a human wriggling around on them.
