The way it works is that these magic items start out with low-level powers and by doing a series of rituals, you can unlock higher-level abilities that ensure the item grows in power alongside the character. In exchange for these magic item abilities, you wind up paying a "personal cost" - a penalty to your attack rolls, skill checks, saves, etc that permanently reduces your effectiveness.
It's a mechanic that makes the items nearly unusable. Balance in 3.5 is hard to judge, but even to the degree that the benefits of the item outweigh the costs, personal costs just feel bad. You get this thing that's supposed to be a power boost and it's making you worse at your job. You get a sweet new +4 sword, but also a permanent -3 penalty to all attack rolls. Or you're a skill-focused character and in exchange for a few low-level spells, you lose 18 skill points.
It's unclear what this mechanic is trying to accomplish. Why give with one hand and take away with the other? Why not give just a little bit less? For that matter, what are we even comparing a weapon of legacy to in the first place, that they require this extra sacrifice? Because a lot of the time, the powers you get are just strictly worse than the spells a full caster is getting at that level. In theory, the reason martial-types don't get high level spells is because they've got superior hit points and attack rolls, but these weapons reduce your hit points and attack rolls and they're still only giving you a 6th or 7th level spell that you can use once per day. The most insulting are the ones that require the sacrifice of high-level spell slots, because they can only be used by full casters, because they wind up being, at best, a lateral move for those who can use them, but would be a much-needed power boost to those who can't.
I suspect what's going on here is that, rather than being excessively powerful in their own right, weapons of legacy break the item slot economy, giving you the utility of a belt, hat, pair of boots, etc, but only requiring a single magic item slot. This can free up your other slots to give you even more magical bonuses. Theoretically, 3.5 has limitations on bonus stacking to prevent this from getting out of control, but in practice, there are so many bonus types that weapons of legacy might well break the power curve. At the very least, they make characters significantly more versatile, which is one of those things that old rpgs greatly overvalued in terms of balance estimates (in fact, one of the major flaws with 3.5 is that it seemed to think a character optimized in one field was equivalent to a character who could do two things half as well).
But picking on a 3.5 book for poorly balanced mechanics feels like a cheap shot. I wasn't coming into this expecting to be impressed. I'm a little surprised at how punitive the personal cost system feels, but mostly the random power levels and weird caster supremacy are classic 3rd edition.
If you set the mechanical issues aside, what you're left with is a collection of distinctive magic items that range from "good" to "very good." Without consulting my notes, there are none that stand out as particularly memorable . . . maybe Merikel, the formerly angelic blade that gained dark powers when its wielder fell to hubris, but that works both ways. There's not a single dud in the entire book. I didn't care much for the spelling of Dymondheart, but the actual blade itself is kind of rad - a longsword made of living wood that has significant defensive abilities (it's strangely not as nature-themed as its background lore suggests).
I guess the Hammer of Witches had some dubious setting implications. There's this sect within the church of Pelor that is all about persecuting arcane spellcasters. Officially, they're heretics, but they still get their priestly spellcasting. It's a little hard to parse, because the shape of the plot only really makes sense if you really lean into the church of Pelor being a transparent copy of medieval Christianity, and then also grant that D&D's arcane magic fills the same cultural niche as European witchcraft. This winds up feeling weird, because in the game's default implied setting, a wizard is basically just a specialized professional, like a blacksmith or a cobbler and the weapon's description backs that up by saying that these guys have absolutely no doctrinal support. So it wind up being hard to even slot the Hammer of Witches into a plot. Plus the name of the item, evoking as it does a real-world atrocity, is a bit too problematic to actually use.
Maybe one dud, then. But the rest fall right into vanilla D&D's wheelhouse of "interesting, but not audacious." You could take any item in the book and build a character or an adventure around it, but none of them would feel out of place with the core.
Strangely enough, while I feel like that last paragraph was a backhanded compliment, that actually reflects more on me than the book. "My problem with this D&D book is that it was too D&D." Let's call it a personal paradox. By my actions, I am undeniably a D&D superfan, but it exists in my head almost purely as a starting point, something to be moved away from. My instinct, when presented with a bunch of perfectly serviceable D&D ideas, is to ask how they can be bigger or spookier or stranger. You give me a magic ring whose ultimate ability is called "pull down a star" and I start thinking in very literal terms. Conjuring an elder fire elemental once every two days isn't going to do it for me (now, stellar elementals, with strange powers beyond the scope of merely terrestrial fires, who are themselves august and sacrosanct in the elemental courts. . . )
If I used this material in a game, I think I'd just completely rip out the personal costs and eliminate all the fixed-use magic items from the core (possibly taking the best of them and using this book's "upgrade a standard item to a legacy item" rules, like they did with the Holy Avenger and Staff of Power). This gives every item in the book a significant power boost, but that should be fine because there are no longer any standard items for them to be balanced against. As an alternate model of how magic items work, the legacy weapons really shine. Each item has a story, and they are all reasonable to hold onto for a character's entire career. Instead of just being random treasure, they are a version of magic items that enrich the fictional world, making it more complex and nuanced, and giving the player characters a tangible connection to its past.
Which, of course, is exactly why this book was made in the first place. Like I said at the beginning, this presents me with certain difficulties. Is Weapons of Legacy a successful book because it provides me with new inspiration on practically every page? Or is it unsuccessful because the first thing I need to do with that inspiration is come up with a new system for how the magic items are supposed to work? Most likely, "success" and "failure" are the wrong terms to use. Weapons of Legacy gives me something no other D&D books, and few other rpg books (even, to a degree, Earthdawn, which had a similar system 30 years ago) can reasonably match. If it's doomed to be just a starting point, at least it has the benefit of being a good starting point.
Ukss Contribution: My favorite part of almost every individual item description was, with few exceptions, the item's "omen." An omen is just a weird magical quirk that indicates the weapon of legacy is no mere +1 sword. Standouts include a sword that roars when drawn from its scabbard, a pair of gauntlets that lets you look into someone's eyes and learn their name, and a polearm that instantly dries you off whenever you emerge from a body of water. My favorite, though, was the Planeshifter's Knife. Set it down on a table and it will flip up to balance on its point. It will spin in the presence of extraplanar creatures, but that function is merely incidental to why I like it. It's just an item that sells its innate uncanniness by casually defying your physical intuitions.
Planeshifting doesn't really exist on Ukss, per se, but I think it will still be an interesting omen, even if the item in question has a different function.