How bad are we talking? Like, compete with MMII bad?
Alright, here' what we've got:
It introduced the Beholder Mage Prestige Class, with all its potential brokenness.
Phaerlin Giant. Challenge Rating 3. 68 hit points, 2 claw attacks +9 (2d4+5 each). Their Reflex and Will Saves suck, but they're a nightmare to fight in melee and will tear up unoptimized low-level groups. An archer on horseback can outrun it and whittle down its hit points, but in enclosed underground dungeons (natural habitat), they'll murder most level 3 parties. Or get caught in grease or fall for a silent image.
Ibrandlin. Gargantuan Challenge Rating 5 Dragon. 135 hit points, +18 melee bite attack (3d6+12), Spell resistance 20. Can also pin, and has a breath weapon (2d6 fire).
Both monsters have not-too-shabby armor class(17), and move slowly, so they're like trolls and giant scorpions in the sense that they wreak hell in close combat for the noncasters.
Tomb Tapper. Challenge Rating 14. Has blindight and burrow speed. Armor class sucks (22), pitiful DR (5/Magic), and its attacks are terrible for a monster of its CR (warhammer +14, 2d8+10), and they have some utility magic (stone spikes, detect magic, and stone shape). Under-CRed.
Greater Doppelganger. Challenge Rating 12. Has 49 hit points, bad saves (7/9/11), pitiful armor class (17) and attacks (slam +7, 1d6+1). Special abilities are underpowered, mostly normal doppelganger except can use any magic item. And it can deal a touch attack against polymorphed creatures and deal damage to them (6d6 damage).
And if it eats the brain of a Medium or smaller humanoid, it can the victim's memories alignment, and all abilities (except 2nd level+ cleric spells, paladin and divine magic abilities). Potentially abusable, but the standard stat block has none of that.