It seems to me that the experience of any one person or the experience that any one person has heard about, for an MB or any other car, will vary considerably. Some people have good experience, others do not.
An objective way to judge the reliability is from some statistical analysis based upon a large sample. The only such analysis published over a long period of time that I am familiar with is Consumer Reports. When I bought my first MB about 35 years ago, MBs were among the best, if not the best, in the frequency of repair ratings. There were very few Japanese cars in the US then. Now, the Consumer Reports charts show MBs as among the worst. The Japanese cars are the best, in general. That's just a fact.
As far as why that happened, it is probably a combination of the standards rising (that is, the best cars are much better now than 35 years ago) and MBs not keeping up, perhaps because of the increasing complexity of the cars.

My own personal experience with MBs has been entirely with cars owned by someone else before I get them. I have greatly enjoyed driving them, and I think their capabilities have risen consistently and considerably over the years. But there have been many times when I wanted to throw in the towel and say "never again." I think every one of them has had at least some repairs that I thought should not have been necessary, or were premature, or were overly expensive. Of the cars I have had, I think the best was my 1992 300D 2.5 Turbodiesel. But I lost it prematurely due to an accident. Maybe if I had owned it longer it would have needed major repairs, too.

I no longer recommend MBs to my friends as a "reliable" car, as I used to you when I was first driving diesel MBs. I think at one point I was able to brag that I had driven somehing like 250,000 miles without being left at the side of the road.

But I still enjoy them, and I keep buying them despite the costs. Call it a passion, or a hobby, or a bad habit. There is something special about the way they ride and handle.
But I sure do wish I could get another 300D 2.5....

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DavidB29
1992 300E with ASR
35 years of Diesels until now!