Faults of the P-38 are very evident. The plane can't dive.
That was it's critical fault. There were other faults, but that was the big one. Completely removed? no, sidelined wrt escort an fighter sweep? yes.
Compressibility was never considered a major issue with the P-38, once pilots were trained on how to deal with it. No, it could not easily follow a 109 or 190 to the deck from 27,000 feet. However, that didn't matter as they were supposed to stay with the bombers anyway. All J models were eventually fitted with dive recovery flaps, and all new L models had them when delivered. The three big issues with P-38s in the ETO were engine failures due to fuel incompatibility, lack of cockpit heat and complexity of the aircraft to fly and maintain. P-38s were doing fine in the MTO, flying from Italy to Germany, Austria and the like without engine failures. Likewise, the P-38 completely dominated the Japanese, flying at medium altitudes (usually under 25k).
At the heights we fly in AHII, compressibility is not much of a problem. In point of fact, if you have any idea how to fly the P-38, it's no problem whatsoever.
I'd suggest that you do some research on the P-38 before telling us what you perceive as "the big" fault.