Time for my two cents.
Yes the m104 leaks oil from the cam cover and the head gasket. If the job is properly done (with the black MB sealer) they do not leak. All of the m103 engines leaked from the cam covers. I saw alot them being resealed at the 15,000 mile service.
The 103 also had valve seals problems. We did hundred's of them and it was not uncommon to have bad cams and rockers. I have not done any valve seals or even camshafts on any 104 engines. I have done a dozen cam/rockers on the new m112 and m113 engines.
Yes, a ton of 104 engine wiring harnesses have gone bad. They could be tough to diagnose because they would not always have the same symptoms.
Ah, but the benefits. As mentioned, the 104 has a nice power curve, and I love the nice smooth idle. I have always been disappointed with the way the 103, and all CIS cars, idle. It was a genuine pleasure to have a smooth idling 104 engine after so many years of less then desirable 103.
Yes, we have seen idle problems from coils (May Christie) in the early production, and plug insulator problems. But the electronic fuel injected 104, whether it be HFM or ME2.1 is without a doubt a great improvement over the time honored 103.
Yes the 103 is very reliable, but the 104 has my vote, hands down. When my 300DT turbo wears out, I will skip past the 103 and go to the 104.