The original South America book had some things so over the top powerful that K.S. pulled it and said "oops, we goofed. This is too powerful." I can't recall any particulars, but there were some crazy powerful aliens that made the Splugorth look like teletubbies.
There was nothing in the South America books that outstripped the Splugorth, or even approached them. A Splugorth has supernatural PS 50 (doing 6d6 MD on a melee attack), 56,000 MDC, 13,000 PPE, 3,000 ISP, regenerated 2d6x10 MDC per minute, had the unlimited ability to teleport across dimensions(!), possession, teleport 600 miles, turn up to 600 undead, animate and control up to 600 undead, summon 2d6 Splugorth High Lords, summon 6d6 lesser minions three times per 24 hours, 11 melee attacks per round, and to top it off, it knew every single Ley Line Walker and Stone Mage spell, as well as every psychic power up to 10th-level. And I left stuff out! The only things in those books that even approached a Splugorth in power were the gods. And even then, they only achieved full power when inside the Great Palace of Omagua, or when in Cuzco. Outside of those places of power, they were drastically reduced in strength, and far easier to take out than a Splugorth.
The weaponry was more powerful than what you found in the main rulebook, sure (one rifle did 1d6x10+10 on a three-round burst), but from what I understand, CJ Carella purposefully did it that way because of the fact that combat in Rifts took way to long (which I can personally attest to. Fighting War of the Four Horsemen was an unbelievable act of masochism which, once begun, you just have to soldier your way through til it's over). So he upped the damage on the weapons.
Siembieda calling someone out for making "overpowered" items is grossly hypocritical of him to do, though, seeing as how he's the man who invented the Glitter Boy, with its 3d6x10 MD gun and 770 MDC armor, and throwing that, the Juicer, and the Dragon Hatchling into the game right alongside the Rogue Scientist, City Rat, and Vagabond. His claiming someone else's work is overpowered is pretty damn laughable. And the way he did it, outright calling CJ Carella out in a game product, was grossly unprofessional. Especially since all the decisions of what to put into the South America books ultimately rested with him. He could've easily toned down any stats that he felt were out of line. But to not do so and then pass the blame onto Carella? When I read that blurb he put into one of his own game books all I could think was "Wow... what an ass."