Engine Issues: The Allison V-1710 engines suffered from compressibility problems during dives
Compressibility has to do with the laws of physics. Due to the P-38s performance being higher than any other American fighter at the time, the P-38 was first to regularly encounter in dives what was then the largely unknown, or at least poorly understood phenomenon of compressibility... loss of control in the transonic zone.
No one at the time knew what compressibility was other than "P-38s can't recover from power dives".
I believe compressibility was the second undeserved strike against the P-38 in the ETO.
The first undeserved strike being the plane's poor initial reputation due to the intentionally crippled engines delivered to the British and French in 1940 which made takeoffs dangerous, destroyed maneuverability and all hope of high altitude performance. The Brits required that the engines not be counterrotating and Washington stripped turbosuperchargers from the order.
poor performance at the high altitudes (above 20,000 feet) typical of European air combat. Turbochargers often failed or froze in the extreme cold.
I believe your understanding about the superchargers stems from British experience with their disastrous 1940 order of crippled P-38s which arrived without superchargers, or counterrotating engines. That established a bad reputation. It was an undeservedly bad reputation.
My understanding is that the turbosupercharger of the P-38 was significantly superior to mechanical superchargers of the era enabling the P-38 to maintain sea-level power up to the critical fighting altitude of 25,000 - 28,000 feet which is where the B-17s and B-24s flew.
Yes, above that critical altitude, P-38 engine performance dropped profoundly but that was by design. As higher altitude variants of German fighter became available, the P-38 had to adapt to fly and fight higher but that is part of the normal progression of variants as each side adapts and improves equipment.
There were teething problems with the P-38 in the ETO but that was the case with every platform. Unfortunately for the P-38, it's high altitude, extreme cold air teething problems were delayed several years due to the 1940 British-French order being stripped of turbosuperchargers.
Mechanical Reliability: Early P-38s experienced frequent engine failures, including blown intercoolers, fouled plugs, and oil system issues in cold weather, leading to high abort rates.
I go back again here to that disastrous British 1940 P-38 order that Washington stripped of superchargers. Had the British P-38s been able to identify high altitude issues, they could have been resolved in the 1942/43 timeframe. At least the intercooler issue could have been been resolved.
I believe the other engine issues were due to differences in the fuel P-38s were designed for versus the British avgas which P-38s had to use once in the ETO. The issue with the British aviation gas tied in with the intercooler issues operating in extremely cold air resulting in the P-38 pilot being unable to slam the throttle and propellers forward as instinctive response to being ambushed. To do so could tear the engine apart and was pilot error. This "pilot error" was solved through a single automated control in a later variant
Combat Effectiveness: While the P-38 was highly effective in the Pacific Theater against Japanese fighters, its maneuverability and roll rate were considered inferior to German single-engine fighters in the ETO. German pilots reportedly considered the P-38 the “easiest Allied plane to shoot down” in Europe.
The initial German pilots the P-38s encountered were highly skilled and flew airframes incorporating multiple combat improvements. As such, there is a certain analogy to the Pacific when inexperienced American pilots initially encountered ultra skilled combat trained Japanese pilots flying upgraded zeroes.
The Americans had to learn some hard lessons about what not to do in fighting the Zero and by the time the F-6, F-4 and P-38 arrived in large numbers, Japans best pilots had been killed by Americans in their "inferior" aircraft.
As I see it, proper operation of the the early P-38 was overly complicated and required a higher level of pilot skill which is why I generally say the P-51 was the best fighter in the hands of the average pilot. Even then, the P-51 as shipped was a dog. How much of that initial low regard for the P-51 was due to the engine not performing well with the British aviation gas I do not know but it wasn't until the British fitted it with a Merlin that was designed to drink British aviation gas that the Mustang became what it was. But by then, the Merlin was a very mature engine.
Yes, an undeserved bad reputation Because of government interference........