I haven't bothered with Liches yet because I don't want my stuff stolen before I'm ready to fight them. But the Parazon is... a bit underwhelming, to say the least. Outside of its plot device function to defeat Liches, it just doesn't do much.
Mercy kills happen very rarely, probably almost only on enemies that you didn't even pay attention to who just got caught in the crossfire or something, like the survivor of a Volt Discharge. Thing is, we generally attack enemies with the intent to kill them and don't stop attacking until they're dead. But the Mercy health threshold seems to be so low that the enemy is gonna be one hit from death anyway. Couple that with the generally fast-paced nature of combat and you'll usually only see the symbol appear for a split second before the enemy dies to your continuing onslaught.
I'm not quite sure how to improve this, though... Simply raising the threshold? Don't feel like that would do. Switching from a health threshold to a different trigger condition? But what would that be that's not already covered by melee finishers. Replace stealth kills in particular? You took away our Covert Lethality for this, so maybe? But that's still somewhat situational and I'm not sure what your vision for these Mercy kills was.
Then we have the Parazon as a hacking tool. It's... alright, I guess? The "Auto Breach" mod is nice to have. Personally, I'm mostly hacking as Operator to avoid vulnerability/detection unless I'm playing Ivara or Loki. Probably just a matter of playstyle, but none of the hacking mods can persuade me to give up the safety of Operator Void Form in most cases.
I was hoping we would also use the Parazon for capturing targets and reviving allies and get mods to improve those actions as well.