As it seems from what you are saying, Conqueror is broken. In reality, its indeed strong but has some weaknesses, its definitely not OP. And the stats you are providing, though true, they are somewhat misleading. Let me explain.
True damage ignores armor and magic resist (you already know that, though). That said, true damage is mostly effective with people having high defenses, but against people with low defenses its weak, far weaker than electrocute. Also, Electrocute offers a lot of burst, while Conqueror gives better sustained. If you play with conqueror and checked on its stats, not just the damage, it shows 2 things, true damage dealt and damage that would have been mitigated, which is in fact the "additional" damage that this rune provides.. If you play against tanks you can see that its extremely effective, since the damage that would have been mitigated is high. On the other hand, if you fight against squishies and people with low resistances you will see that the damage that would have been mitigated is so low thats its not much worth. If you test conqueror and electrocute against someone with low resistances you will see that conqueror offers little to no value. Its not the actual "true damage dealt" that matters but the damage that would have been mitigated, this is the real additional damage the rune provides.
Also, true damage is, in a sense, the counter-part of physical and magic damage. Physical and magic are "countered" by resistances, while true damage is "countered" by health.