Mostly true. But check a p2pool website (http://freebtc.eu:9173/static/). You'll see doa% for the pool is ~20%. Those % do not count in lost hashrate for you. Only the doa% for the miners count, and everyone has a higher % on p2pool so your share from the profits are not affected by that (but do try to get your % down, this will up your valid hashrate).