Balance discussions revolve around what’s strong and what isn’t. Saying it’s possible to build something niche does not prove anything here. It’s also possible to finish main game till AoM, choosing no mastery at all. It doesn’t prove jack.
Most arcanist builds are (comparably) weak, and there’s no two ways of saying it. IEE is weak and supports stuff that’s useless for mainstream arcanists. Star Pact is great for the cdr but only supports cold. Finally, the numbers in the mastery do not make up for no rr, FACT!
Just check any ranking of crucible runs or SR achievements. There are no arcanists there. There was one but was quickly put in its place next patch. Why?