I think the main problem with the HT4100 was simply lack of development before it was released. Then it was up to the hapless consumer to pursue the development in subsequent years.
Perhaps, the reason that the engine was released before it had been adequetly developed was the tide of political and social events at the time. Fuel was still at a premium, Cadillac had failed with the V864. There was a demand for fuel efficient cars, and the HT4100 was very fuel efficient. So, it was relesed before it should have been.
Also, Roger Smith being the consummate bean counter, saw that further development was unecessary and costly. of coarse we all know what this man did to the reputation of General Motors as a whole. They are finally digging themselves out of the mess he created.
The design was sound. Witness the reputation of the 4.9 V-8. It is a very durabe and reliable engine. It makes good power and returns excellent fuel economy. Disimilar metals have been used for decades in engines with good results. It was the small things, like the casting techniques of the block, poor head gaskets. Main bearing knocks that could have been prevented with more judicious testing. But others like camshaft failures caused by both poor material and defective oil pumps are utterly common components. No excuse for that. Simply cost cutting.
These were the dark ages for GM. Yes, some very good and durable cars and trucks were built, but there were also many very horrible cars built. The early J-cars were pure and simple, junk. The Camaro and Corvette, while touted in their day, are compromised designs. An on and on. Perhaps the jewels of the GM fleet were the full sized RWD cars. Simple, rugged designs. From these, the C and D body Cadillacs were built. The E-bodies were a technological tour de force, introduced long before Roger Smith could ruin them. They were good cars too. Save for the problematic engines. Then there was the Fiero. That one is best left for its own discussion! Saturn, Smiths baby, was always trailing the pack. Its innovative dealer network is more or less a bean counters delight. While they have proven durable, they are certainly not inspring cars. In the early years, Saturn was a financial sink hole for GM. The primary reason for GM finally seeing the light and disposing of him.
But as bad as some of the stuff GM was building, Ford and Chrysler were making them look good. The endless K-Car iterations form Chrysler. Rough, noisy and unrefined. Then Ford had the Tempo and Escort. They pounded out an incredible array of variations of the plebian Fox (Fairmont) platform and only recently saw fit to bury it. They based everything from dope dead econo boxes to Lincolns on this platform.