Take the ninja, for example. In isolation, this is a fun class. In combat, you can run around the battlefield, striking from the shadows. Out of combat, you're this super-sneaky infiltrator with magically enhanced stealth abilities. One of the classic fantasy archetypes, brought to the game as a 20-level standard class at last.
Their capstone ability, gained after 20 levels of single-class progression, is called "Ghost Walk." It allows you to spend some of your ki points to cast the spell Ethereal Jaunt. Ethereal Jaunt is a 7th level spell. Wizards and Clerics get access to it at 13th level. Now, granted, a wizard or cleric isn't guaranteed to have Ethereal Jaunt accessible, because they've got a limited number of 7th level spell slots - a minimum of 4 at 20th level - but that's hardly a recommendation for the Ninja class. If a spellcaster did not have Ethereal Jaunt available, it's because they're using that resource for other things of comparable value. The Ninja will most likely always have Ethereal Jaunt available (I figure at level 20, you've got enough ki points to use it 7-8 times per day, assuming you do nothing else with your points) because they have nothing of comparable value.
And that's not even the most insulting of the Ninja abilities. That would be "Ghost Step" which allows you to spend 1 ki point to become invisible for 1 round. Given the timing rules, it has certain situational advantages over casting the Invisibility spell, but you get a lot fewer daily rounds of invisibility as time goes on, and no increase in functionality comparable to Greater Invisibility (well, it does upgrade to make you ethereal as well as invisible for 1 round, but 1 round of etherealness is . . . extremely situational).
Now, the tenor of my complaints may lead you to think that I dislike the Ninja class and think it's terribly underpowered, but actually the opposite is true. Wizards and Clerics are some of the worst-designed classes in the game. But their faults run towards being bland and overpowered. Ninjas are the magical stealth guys, but spellcasters are better at being invisible, better at being ethereal, and even better at jumping (the jump spell at level 1 gives a +10 bonus to jumping rolls, a ninja's Great Leap and Acrobatics abilities give a combined +10 bonus jumping rolls at level 18, by which time the jump spell has improved to providing a +30 bonus). You could argue that, theoretically, a wizard isn't going to be specced to obsolete a ninja, and really, a +30 bonus to jumping looks impressive, but it winds up being a smaller modifier than the 18th level Ninja's 21 ranks in jumping + 10 bonus. And on a full adventuring day, maybe that wizard is going to run out of jumps, whereas the ninja can just jump constantly.
And all of that is even more or less true. But it is a curse knowing that this interesting class is not the best possible build (or even, really, part of the best possible build) for its signature tricks.
Also, WotC, what's going on with the weaksauce +2/+4/+6 Acrobatics class feature? Was there something wrong with the Bladedancer's +10/+20/+30? Are you really being stingy with niche traversal abilities in a game where Fly and Teleport are options?
But that's kind of Complete Adventurer all over. I really dig its vibe, I like pretty much everything I see, the Main Character Energy is off the charts, but you're a fool if you use this book to build a character.*
*Some of the spellcaster-focused Prestige Classes will work all right.
Ukss Contribution: Oh man, you have no idea how spoiled for choice I am. There's a prestige class that lets you use stealth with your horse. A conspiracy of freelance spies whose goal manipulating various nations into peace. The Ghost-Faced Killer prestige class, which made me do a deep dive to figure out why they named a class after a rapper, which led me to learning of the existence of a 1979 kung fu movie called The Mystery of Chess Boxing. The suggestion that I could adapt a good-aligned prestige class that focused on hunting evil by changing its alignment and coming up with a more sinister name than "Shadowbane Inquisitors." The Vigilante prestige class having a signature character called "Beasly 'the Nightstalker' Bigums." Minotaurs making musical instruments out of the horns of minotaur heroes.
I just absolutely love so much of what this book has to offer.
In the end, though, I have to go with ninjas. I really like that Complete Adventurer was willing to go there, I just kind of wish that D&D 3.5 was the sort of system that allowed it to really go there. And don't get me wrong, I don't blame the author for not making ninjas fantastical enough - it is entirely the fault of D&D's weird idea that breaking verisimilitude, even in a small way, is a high-tier power (but also, somehow, spellcasting does not break verisimilitude) - but I yearn to see them in a setting where they can be allowed to flourish.