Having read that book, I have the following to add regarding preparedness: They had body armor, and they had long arms. They just didn't bother to prepare themselves to use that gear, because they seemed to believe they would have time later in the morning. Clearly wrong.
Marksmanship was obviously a huge failure, which is why I originally asked if they were using .357 magnum ammo. If they were, it could have explained why those with revolvers failed to make good hits up until the getaway attempt. But that was evidentially not the case.
Tactics appear to be an even bigger failure. Only one person brought a shotgun into the fight, and he ran head first into rifle fire in an attempt to get there quickly. Consequently, he was shot early on. One other person attempted to retrieve a shotgun, but did so once Platt was mobile, and got shot a paralysed for the attempt. A car that arrived after the fight began, with two agents inside, parked 45 to 50 yards away, so they could fire from the cover of their vehicle. No attempt to get closer, and no long guns were brought into play by them. Three guys huddled behind one car, all lost sight of Platt at the same time due to lack of communication on reloading. All three were gunned down when he came around behind them.
So yeah, massive failures other than ammunition choice. And apparently shooting .38 Special +P rather than .357 Magnum didn't reduce recoil enough to improve marksmanship a whole lot. Except maybe for Mireles at the end, shooting one handed.