Only issue is that a lot of the feat trees build off core stuff. Good luck getting any offensive feats if you don't have Power Attack, for example.
Yeah, that's the one hard part about it. Luckily, the ToB classes don't actually need the Power Attack lines, so it's mostly okay. Chargers do get hit HARD though. And obviously trippers aren't great, but again Swordsages can make that happen, and War Hulks can make do as well.
You do have to lose some things, but it does come out much more balanced.
And yeah, Andy is quite right. They did playtest a lot, but they didn't really understand what they were doing, and it shows. They were told to look for what was powerful, but all they saw was that damage was powerful, hence the whole overestimation of damage spells while totally ignoring spells like Glitterdust.
And they completely didn't think about stuff outside of combat, thinking combat was the only important thing about balance. Basically, they playtested for kick in the door play where doing hitpoint damage was all that mattered, without ever really using buffs of any kind. Just damage the enemies and heal your friends (yes, the Clerics in playtest were healbots).
Interestingly enough, core is TOTALLY balanced if you play this way. Core only, a Blaster Wizard with a Blaster Sorcerer, a Healbot Cleric, a Druid that never Wildshapes and just heals and blasts, and a grouping of the rest of the classes who focus entirely on just hitting stuff and healing come out completely balanced. Weak, but balanced, though strength becomes the really important stat (yes, Half Orc Barbarians become the most powerful class!). That's actually how the game was intended to work at first. Note that the CR system works a lot better there too. Of course, the game is pretty darn boring that way, but whatever. As soon as creative people took hold, they realized that damage wasn't the real priority, healbotting was inefficient and pointless, out of combat power was as important as (or more important than) in combat power, and so on. And when you do that, Core is just stupidly broken.
JaronK