Most electronic artists sound dated rather quickly because they are really working off the cutting edge and experimenting on it heavily. Not everything works out and the things that don't get stuck in that date forever, but then there are others who still sound like they are in the future.
I have found the opposite to be true. Eno and similar artists like Harold Budd sound dates because they use real world instruments like piano. The electronic artists sound fresher, although of course there are exceptions.
Deicide through a kazoo, does it change genre?http://www.youtube.com/watch?v=vygsU314TaA
The ambient mostly consisting of electronic artists like Aphex Twin, Autechre, Lustmord, Gridlock, Biosphere. I think some of the old schoolers like Brian Eno sound kind of dated. I discovered ambient music before I learned that black metal artists were evolving into that style, but came to appreciate that too.
Again, we need to separate instrumentation from musical composition.
If I play Bach on guitars with a drumkit going in the background, does it magically become rock?
In the same way, if I play Bob Marley through a symphony, or Deicide through a kazoo, does it change genre?
It doesn't: genre is composition.
For ambient, look to Tangerine Dream and Kraftwerk. See if you can spot any structural similarities -- when do riffs change, how do they change, how is melody used to change the song from start to finish -- between those and death/black metal.
I was having this argument with myself yesterday: Does timbre matter? Yes. It's probably secondary to composition, but still relevant. If you were to play Bach with drums, it would change the composition. Similarly, this (http://www.youtube.com/watch?v=16zL4VMzs4g&hd=1
) is just guitar leads, so it changes the composition. Metal has a lot of percussion, ambient has very little. Ambient is often used as an adjective, so you could apply it to metal (HLTO), but metal has more rigid rules about sound and instrumentation. I would say you can't even call it metal if it has acoustic guitars. On the other hand, you could call genres restrictive constructs etc. and not even necessary: it's music in the end. Even if composition is more important overall than instrumentation, instrumentation is more important to determining music's genre, but then again genre really isn't that important. I know we would like to afford metal a transcendent sort of status, but it depends on the context. The difference between metal and rock is probably the best compositional difference, although you have to consider how distorted the guitar is too. On the other hand, Nu Metal really is metal, just bad metal.
Regardless of how important you think composition vs instrumentation is, metal and ambient have different compositional techniques too. Comparing metal to ambient, ambient has a more textural approach to music. Metal has more discreet parts and ambient is more amorphous. Tangerine Dream for example uses repetitive riffs and changes the envelope to change how it sounds. I loved Phaedra but the rest of the discography is pretty redundant. I tried to like Kraftwork, but I don't think that's happening.
Some other ambient artists:
Aidan Baker (associated with metal via Nadja)
Bohren & der Club of Gore (started off as grindcore)
Final (Justin Broadrick)
Maeror Tri (thanks ANUS)
Nordvarg/Drakh (black metal inspired)
Stars of the Lid
Vinterrikket (black metal inspired)