If you compare resource efficiency of archer immortals relative to CS archers, they are the single most OP champ of the game. However, they are still archers, so they take a little more time (you need a lot of them) to really be a problem.
From my calculations : immortals archers are way OP, but spears are only slightly weak.
Now, because they can switch from one mode to the other, this make them extremely op, because they are archers... that can counter archer's counters : cavs. It's a fun mechanic, and I wish they were more dual weapon units. However their balancing is failed, as they should have instead of being strongest archer of the game (in resource efficiency therm, once again) be weaker, to balance dual weapon.
Also idk why archer and melee stance have the same hp, probably transform.js bug or lag if you rescale hp too often but pretty sure it's supposed to handle hp rescales. @real_tabasco_sauce