Requiring the Feat and Time & Money requirements for the rituals would probably be sufficient. The drawback to so many of the weapons are at the end of their progression the weapons wind up not being truly the plus they should be.
At 20th level having an Intelligent +4 Keen, Holy, Flaming Icy Burst weapon would not be all that unusual. Now take a Weapon of Legacy item, Randomly I just opened the book to Caladbolg on page 48. Caladbolg is a short sword that ultimately becomes a +4 mighty cleaving short sword that grants a +6 enhancement bonus to STR,
a +4 bonus to bull rush opponents,
a +2 resistance bonus to saving throws,
a +4 bonus on the attack roll to sunder an opponents weapon and
+4 to the damage Caladbolg does to objects
Unstopable Cleave...you can make an extra attack w/ cleave even if you did not drop a foe
and the best ability: Imprisoning Stroke...once per day you can use the Imprisonment spell on someone you hit...standard announce you are using the ability before you roll rules apply..and there is a Will Save.
A pretty good list of abilities, that on their own, w/o penalties would be worthy of an Artifact. Now just using the standard feat rules, you would have to invest 3 feats in order to unlock all of these powers...which is a pretty hefty investment on it's own.
You would also take (at the end of the progression), a -3 penalty to attacks, a -4 penalty to Reflex saves, and lose a total of 10 Hit Points. In essence the sword becomes a +1 sword, the situational bonus to Sunder and Bulls Rush are reduced to +1, the bonus to Saving Throws reduces the Reflex save to a -2 penalty...but you very well might never use that bonus because a Cloak of Resistance +5 is almost de reiguer.
You ultimately wind up with a +1 Mighty Cleaving Shortsword, that is not quite as effective as a Shatterspike weapon at cleaving, gives a modest bonus to bull rushing, acts like a Belt of Giant Str, (not bad), and can cast Imprisonment once per day.
Depending on how you decided to treat the Imprisionment ability under the Magic Item Creation formulae, this weapon is reasonable to create as a top tier weapon, w/o investing 3 feats and taking other penalties.