Now this sounds like a proper explanation, i can buy this.. all day over the safety concern BS
So rumors of failure are in full swing. My personal favorite is that McLaren's hybrid system isn't capable of providing power through the whole 12.9-mile circuit. This post on the the seriously devoted McLarenLife forum explains that the Porshe 918 may have more battery capacity than the P1. Beyond that, the 918 may be better at recharging its battery than the P1. The 918 can recharge its batteries with its brakes, whereas the P1 can only recharge with engine braking, as this other McLarenLife post points out.1
I've heard personally that the P1 might require two recharges for it to deliver full electric power over a single lap of the 'Ring.
Many on McLarenLife presume the car is running hot laps sector-by-sector rather than attempting full laps.
German engineering wins it again

.. according to rumors Ferrari is also struggling with the KERS..
Big up to Porsche!
PS
THIS!
Horacio Pagani, who arguably builds the most 'super' supercars in the business these days, was right when he said that the P1 is missing the emotion you expect from it. Its carbon tub is too similar to the one in the 12c. Its engine is too similar, too. It's not as pretty as the old F1.