All of your conjured creatures are extraplanar beings that you're summoning to this Plane, binding to do your will, and have a solid chance to kill you if you screw this up and/or piss them off enough after binding them.

Familiars are sort of the opposite. You're summoning a much weaker creature, not nearly as powerful for fighting, but one that won't even kill you or even attack you. Instead, most of them have grant perks to your character (extra detects, able to "sorta" be in two places at once) - and some even more perks when directly in your presence. In exchange, they themselves are often a vulnerability that you now have to protect when enjoying these perks. Power word kill, with that particular level difference, tends to become the most obvious vulnerability.