I've noticed that the V8's don't suffer the head gasket leaks like the M104's.
They also get almost the exact same gas mileage, only 1 mpg less in city / highway driving than the M104 engines.
I don't think they have the same high rate of engine wiring harness failures, but there are 400E / E420's that have had their wiring harnesses replaced.
I've noticed, though, that although the head gaskets don't leak, the timing chain / guide rails are recommended to be replaced every 100-120,000 miles or so. The timing chains are very long (about 6 feet) and stretch over time.
The timing chains on the M104's is very short, and the techs say that the chains on these engines never need to be replaced.
The timing chain issue has been discussed a lot on this site.
If you do a search on "timing chain" you'll probably get a lot of hits.
Also, even though the badge on 92-93's says '400E', it is the same 4.2 liter engine as in 94-95 E420's.
So, you may need an engine wiring harness regardless of the M104 or M109 engine, and the need to replace the head gasket on M104's is offset by the need to periodically replace the timing chain on M119's.
If it were my money, I'd look real closely at the 400E / E420. You get more power with almost no loss in mpg.
I'm not suggesting, though, that the M104's are underpowered.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".