Now Phantasmal blades in its current state is a fantastic shotgun skill.
I did however notice that the existing transmuter’s conversion from pierce to vitality is hindering build diversity.
My suggestion is to keep the pierce to vitality conversion a separate transmuter so that anyone willing to focus on Pierce Phantasmal Blades or Cold/Pierce blades is allowed to do so.
I’m personally not a fan of the way the transmuter works right now, but I think the bigger obstacle to build diversity in general is poor itemization.
EDIT: To clarify - I don’t like that if you want a spammable skill, you have to go vitality damage. In this sense, I agree with the OP and agree that the transmuter hinders build diversity.
EDIT 2: I also don’t like that the “spammability” transmuter actually only really makes the skill effectively spammable if you have EXCELLENT e-management, but that’s a whole different can of worms. (To clarify a bit here - the reason Spellbreaker is the go-to class combo for Pblades is because of the e-management afforded by an Arcanist sub. Spellbreaker actually has less damage synergy for the skill than, say, Trickster - Trickster has absolutely perfect synergy for Pblades or Pblade spam but it has lousy itemization for such a build and poor e-management, making it much less attractive an option than it should be.)
EDIT 3: (Last one I promise): I am personally of the opinion that modifiers and transmuters absolutely need to be more “modular” and compartmentalized in their effects, in general. Sometimes the “bundle” idea works well and contributes to builds and build diversity, but only sometimes. I also think that certain modifiers, transmuters and even flat-out skills need to be made available contingent upon what class combo you opt for (like, say, skills/transmuters/modifiers that only show up in the skill trees when Trickster or Witch Hunter or Spellbreaker or whatever is the class combo in play).