I'm not entirely clear on those legacy feats. The way the section is worded it looks like you get those as bonus feats.
As for the advancement costs, I think they make a lot of sense. You get a lot of abilities as the weapon advances. If it was layed out as a prestige class (like those in the Game Mechanics' Artifacts of the Ages series) you would probably end up taking a hit in some combination of hit dice size, bab, save progression, skill points, class skill list, and class abilities. If the progression had been built into a prestige class a doubt anyone would complain about taking an equivelent hit. Okay, someone would have complained, but to the majority it would have been clear that you don't get something for nothing.
I like this solution, you pay similar costs to what you would by taking a prestige class to get the abilities, but you can continue to stack up class benefits for your chosen core or prestige class while still unlocking the weapon of legacy's powers.
Morrow