I've had 4 diesel vehicles in my name. Here is my summary of quality over the decades.
W123 - My '85 300D and 83' 300CD cost about $35,000 new when all other cars like Toyota can be bought for $9000. The W123 & other MB cars were by far the most reliable luxury vehicles money can afford. No other brand can say the same. All cars Mercedes put their name on had the same consistent reliability all across the board. Those cars I bought all had 300,000+ miles and will last more than 50 years with regular service. The only problem to look out for is rust. My 300CD
was my favorite and had little rust.
W124 - Launched in 1986. This line of Mercedes was lightyears away from W123. Aerodynamics, crash zones, airbags, electronics, soundproofing and performace never seen before on the w123. Gone are the shiny chrome. This is a very different era. Prices still hovered around $35K. Toyota, equally making reliable vehicles sold cars for around $12,000. My '86 300D had the Om603 6-cyl engine. This was the fastest diesel Ive ever owned. I also consider this engine reliable like the OM617.
W164 - I bought this for my wife who hated all my 80's era MB "diesel collection". I hoped to make her a believer of MB diesels. The ML I got has the same OM642 used for light utility trucks and E-class diesels. I still consider it reliable except for the time i snapped a serpentine belt caused by dripping oil into the belt.
Nowadays, the C class now costs around $40K. The A-class starts at $30K. Not much, difference in prices from the 80's. But adjusted for inflation - the prices for MBenz cars really dropped considering a Toyota / Lexus now costs about the same as a Mercedes. But the biggest difference is - reliability of other makes/models. There are several other brands that can easily compete with Mercedes. So there is no more Toyota/Mercedes monopoly of reliable cars. Sadly, Mercedes lags behind Toyota in sales and reliability. Even in basic "rustproofing" - I am disturbed by the sight of all those rusty C & E class cars on the road. I hope Mercedes wakes up and go back to fixing what matters.