You're confusing the fluff with the mechanics.
The fluff of the truenamer is awesome, taken from all sorts of stories1
Unfortunately, the Truenamer handbook shows us that this brilliant promise of narrative isn't particularly well supported by the rules, which were likely unplaytested and only lightly edited. This mainly stems from two of the class features which suffer from lack of choice, poor scaling, and a few really horrible class features.
In summary, imagine a warlock with less choices who had to make an increasingly harder roll to contribute minimally throughout the day. Yes, it's possible to make one barely functional, but with the same effort dedicated to any other class, the class can excel in what it was designed to do.
On personal truenames:
What kind of bonus? Well, pg. 200 lists it as untyped, and pg. 196 says it's a competence bonus. Beats me.
This is an easily seen example of the lack of care or editing put into the class.
[Truespeak is] A trained-only INT-based skill that you'll need to keep maxed at all times. Pretty much everything a Truenamer does requires a Truespeak check, and the typical DC is 15 + (2 × CR) of the target. Yes, this means that the typical DC goes up by 2 every level, while you can only add 1 rank every level. This is just as annoying as it sounds, and it means that you'll be spending a disproportionate amount of time trying to boost this check . . . then trying to figure out what to do with the check once you've boosted it.
Therefore, most of the character's resources will be going towards optimising a single skill that, without that devotion, is completely unusable and scales even worse. The fact that it scales with target CR, which tends to be a ... not entirely random number... just compounds the problem.
The "item changing power" also suffers from this odd scaling:
The DC to speak one of these is 15 + (2 × CL), where CL is the caster level of the item. If the item is nonmagical, the DC is a flat 25. Yup, that means that you'll have an easier time affecting the little trinket that the apprentice mage enchanted than the nonmagical thing he started with.
The two Law of X class features: the name of "suck"
Law of Resistance (LoR): The first Law of WotC Hates Truenamers, this is an annoying little rule that makes Truenaming harder as the day goes on. All those utterance DCs I gave you above are just for the first time you use any given utterance during the day. Each time you succeed, the DC of that particular utterance increases by 2, though (in a rare display of mercy) failing doesn't increase the DC. Yes, this is kind of a pain to keep track of. Anyway, I think this is intended to keep you from just using your utterances at-will, but it basically means that low-level or unoptimized 'Namers will have a hard time doing anything past the first combat of the day, while optimized 'Namers will basically just ignore this until they actually have to roll to Quicken. It's still annoying.
Law of Sequence (LoS): The second Law of WotC Hates Truenamers, this Law will be the bane of your existence. The LoS says that you can only have one "copy" of an utterance active at any given time. This means that if you have, for example, Knight's Puissance active on your Warblade buddy, you can't cast Knight's Puissance again on your Crusader buddy until the first one runs out, nor can you cast Reversed Knight's Puissance on the Bulette you're fighting. If you've never played a Truenamer, you might think that the LoR is worse than the LoS. You'd be wrong. I consider the LoS to be one of the single worst-designed parts of the entire Truenamer chapter, and you can quote me on that.
Thus, in a class designed around a few repeatable effects, every time the class tries again throughout the day, regardless of target the effects are harder to pull off. Think of a psion who has to pay extra power points after the first use of a power in a day. This was intended as a "limitation of spells per day" ... but no evidence is given to show that it actually managed to adventure and contribute in a group.
At the end of the day, there are no workable mechanics for the truenamer, and the truenamers require huge amounts of optimisation to even be "playable." They cannot "do one thing well" and without weapon or armor proficiencies, nor hitpoints nor a good BAB, can they even readily contribute to combat. If, at the end of the day, anything a class may want to do may be done, better, by another class... the class is poorly designed.
See A Wizard of Earthsea