Unlike the copper projectile of Deep Impact,
The (Philae) lander structure consists of a baseplate, an instrument platform, and a polygonal sandwich construction, all made of carbon fibre. Some of the instruments and subsystems are beneath a hood that is covered with solar cells.
An antenna transmits data from the surface to Earth via the orbiter. The lander carries nine experiments, with a total mass of about 21 kilograms. It also carries a drilling system to take samples of subsurface material.
I think they're landing before the comet gets real active. The lander is mostly non-conductive carbon fibre so the electronics should be OK...at least until the comet begins to discharge, increasing the vulnerability of electrical systems. I wouldn't expect any discharge activity when the harpoon anchor hits the surface either.
My concern is that, after activity picks up, if we're fortunate enough for a local discharge to occur around the landing zone, the instruments might get fried preventing us from getting any data on the event. Regardless, I wouldn't be surprised if they have ongoing issues with the electronics and/or communications between lander and orbiter as the comet gets closer to the sun.
The European Space Agency, Rosetta mission homepage
Rosetta factsheet (pdf)
Thanks for the nod Mike, but I stand humbly in the shadows of those giants.