On bike I have mostly seen chains dismantled by lateral forces, or if a pin is badly installed it will slip from one end and will bend out one sideplate with the pin attached. An interesting chain break that I have seen was a fracture in a sideplate (and bending out of the other sideplate still attached until it removed the pin from the plate).

As for fixed gear forces.. they are considerable higher than in any other bike, because of the stiff transmision. If the pedal hits something.. it's bad, if you hesitate pedaling a bit when coming of a kerb, if you frequently skid brake (and the skid is unsuccessful because of a high grip surface) . Inertia is a huge problem (inertia of the human that bounces on the pedals on various occasions), and the energy of the force impulse .. it's big. Plus all the forces go both ways on the chain in drive or brake mode
- Just my common sense, no numbers.

I do agree that chains are fairly safe on fixed configurations with no shifting and no lateral forces, but depends on the redundancy available on the bike (like braking.. on normal setup we have two brakes, in some odd setups we have no brakes and one chain.. how good that chain should be as the main component in braking? enough to ride without constant worries and fears.. or I'd better put some brakes, but it's another story)

In industrial chains, lots of chains break at the pins (some of the pins bend under the traction force and acts as a stiff link and in short term that pin will break) - but industrial chains are usually under constant load, constant speed, well lubricated being submerged in oil all the time.

Conclusion: for the peace in my mind I go with the full-bushed chains 1/8" made by yaban (yet the manufacturer is not so important)