The problem isn't that Cobalt soul is objectively bad design it's the whole line up. Chrono wizards are busted because of lack of understanding of a major restricting limitation of spell casting, echo knights is just word salad at its best, gunslingers is a more limited battle master, .. oath of open sea is ok but it's hard to miss with paladin oaths, runechild missed the fact what features a sorcerer has at level 1, hemocraft is ripe for abuse, juggernaut reinforces that barbarian should be avoided, blood hunter is getting there but it's still full of issues.
5e is robust enough that you can get away with some bad design but it doesn't take away from what it is.