I believe we're thinking on two different tracks here. I didn't say Warforged make exceptional infiltrators, just that they aren't poor infiltrators. Yes, they have weaknesses, but they also have unique strengths to bring to bear also.
First, Invisibility doesn't need to be on all the time, just getting over the wall or through the gates when they open. Competent infiltrators will have many ranks of Hide and will find good places to hide: Warforged have no trouble with that, mechanically. Light is typically not an infiltrator's friend, regardless of race, so naturally you'll try to strike during the night, when the sun isn't glaring at you from above.
Pertaining to not sleeping and walking under a river, I speak specificly of walking all night underwater without comming up. Potions of water breathing only help so much in this case, especially when you consider what happens when you open up a bottle underwater... and not being restrained by sleep cycles, you can walk all night without a problem, except for a bit of nonlethal damage you'll eventually occur, which can be easily solved with a UMDed Wand of Repair Light Damage.
Oh, and reeds don't help either when you're at the bottom of a river, rather than the shallows. And what can I say, people are suspicious of reeds that move, after all.
