From my study the Army was actually rather successful in their amphibious ops.
Prior to WWII it was almost an article of faith that an amphibious operation against an entrenched, well defended beach was impossible. The proof was the British failure at Gallipoli. Actually that "proof" was highly questionable. The Marines during the '30s, in conjunction with the then quite small Gator Navy, did question that "proof" and came up with a plan that could work. It was put into effect during WWII. As with any new plan there were complications. Early experiences, as at Tarawa, were not always happy ones. Fortunately, we didn't "hang" the senior officers responsible for the planning but rather let them learn and future ops were much more successful.
The Army watched and learned and they, too, became quite adept. In the Pacific those ops were mostly rather small (until you get to the invasions of Guam, Saipan, and the PI). In the Atlantic/Med they were fewer in number but very large operations.
The old saying that "amateurs study tactics and professionals study logistics" is never more true than in amphibious ops. One major difference between the Atlantic and Pacific ops is that the only really "long range" LANTFLEET invasion was Operation Torch in North Africa. All of the others were launched from secure bases with relatively short lines of supply. In the Pacific I can't think of one (except, maybe, for some of the ops in the Solomons and surrounding islands) where the line of supply wasn't hundreds, if not thousands, of miles long.
The Army also had, in its background, the operation against Cuba. Frankly, it was a Charlie Foxtrot. The Army in 1898 had no experience in "out CONUS" operations. The Navy was not a whole lot better, but at least had thought about it and had a plan (however untried). The Army was frankly embarrassed badly by their logistical failures and really did start to think about what they needed to do. Not only did they have to supply Cuba, they needed to support an operation in the PI (that would drag on for almost 13 years). This was the OJT that would give them real world experience in combat support. It would pay big dividends in WWI and really come to fruition in WWII.
The Army's biggest PHIBOPS failure was Anzio. It was a good idea that went wrong when the ground commander failed to aggressively push forward and seize tactically crucial positions. One reason given for this failure was that a major portion of the sea lift to support the operation was scheduled to be transferred from the Med to Britain as part of the build up for Overlord. I'm not sure I buy that, as the tactical surprise achieved was pretty much total and the "window of opportunity" was wide open. Yes, it would have been a risk to put rather small forces out front and then work like Hell to back them up but the alternative, which happened, was to let those crucial positions be occupied by a resourceful enemy. In reality the Army paid a heavy price for what amounts to a failure of aggressive leadership. Another hallmark of PHIBOPS is that they really do tend to be "all or nothing" affairs. The failure at Gallipoli was similar.
SQQ