I doubt it's mechanical failure, but maybe. Still, it's the first F15E that has fallen down during combats.
I hear the US is using B2 airplane worth 2 billion dollars that carries 6 (?) nuclear missiles. Isn't that a big risk, what if they take it down?
The B-2, or any aircraft for that matter, do not carry nukes unless it's on a training mission related to nuclear deterrence or if they actually plan on using them. Their main base is in Missouri, and they would not fly +20 hours to attack Libya while carrying weapons they have no intention of ever using. They take up unnecessary space, and limit the amount and variation of the payload they actually plan on using.
So were F117A but they were shot down.
Different aircraft, with a variety of different performance characteristics, not the least of which pertains to their "stealth" capabilities. The B-2 is an evolution in that regard, and the "stealth" technology employed on it is different from that on the F117. Just to clarify something, "stealth" does not mean invisible. It deals with the range the enemy is able to detect it, and from there the ability of tracking radars to maintain a consistent picture, mapping its flight path. With stealth aircraft, it is extremely difficult, and the strides made in electronic warfare since the 1990's (a field in which the Americans lead the way) make it even harder.
As for the F117 that was shot down in the 90's, no1 knows for certain what happened. There are an endless amount of rumours as to how it was shot down. What we do know for sure though is that the hit was successful because of the tactics the Serb SAM crews were using, not because of any technological deficiencies in the F117 (as demonstrated by it successfully destroying more heavily defended targets during the war).
For example, they never left their various radars on for long periods of time, switching them on long enough to gain a picture (or confuse the enemy as to their real locating), turning them off and then relocating them to another position. In any case, the Americans learned from w/e mistake were made that day, and have devised counter-measures to those tactics.