Friendly reminder, low pick rate =/= balanced

Low pick rate doesn't mean balanced. High pick rate doesn't mean imbalanced. Neither means the opposite, either. They are informative, though, particularly as a factor when interpreting the meaning of win rates.

low win rate =/= weak

I don't know. Nasus has a pretty low pick rate, and in this current meta he's pretty imbalanced. I can't tell you how many games I've recently gone with 0 deaths up to 2 deaths, with 11-16 kills and being neigh-unkillable in team fights with the new Grasp of the Undying mastery, Spirit Visage and Nasus' passive.

Pretty much Heimerdinger.

I would actually argue that low pick rate = unbalanced more often than not.

aka pre-reworked poppy.

tbh a lot of the low pick rate champions have been intentionally gimped by Riot at some point in time. Usually champions who are pure melee or have hit-or-miss skills that either break the game or do nothing meaningful whatsoever.