Don Passione is absurdly strong if opponent has no debuff removal, perma immunity etc. and lets you stack 5 Miasma. She also has an innate Immunity which works 75% (with max Accuracy) of the time with BLEED, HEX, QUIETUS infiliction on top of it, but I wouldn't count on them, think of them like a bonus. However most important thing is that she is an Eliza. When you combine good SAs with Eliza's kit, you get an insane unit. 5 Miasma + Heavy Bleed + normal Bleed(s), they can't physically live long enough to let you stack them all. She can even retain those Miasmas if she immediately tags out after defeating the opponent, so using units like UHost as a bridge works incredibly well.
Next ones that currently we know are Shear Force and Malice.
While Shear Force has one hell of an overloaded kit, I think I still would prefer Grin Reaper solely for permakilling enemies in rifts, as she removes the corpse as well. For PF however she should work significantly better due to her insane damage potential. I still kinda want her but I'm not sure if she can even add any utility to my collection at all.
Malice is an incredibly flexible unit, pretty much like Harlequin, she is both a support and a DPS. I didn't see myself using her as DPS in rifts though as Heartless already covers the part she is supposed fill in (Last opponent killer). Also most of the time you don't need to increase the already hard hitting crit-based units damage in rifts as HP multipliers aren't that challenging. There are some exceptions however, for example when you make a team of Malice + Seraph + Blue Streak (with max crit rate) you want her to get the Enrage asap so she can enhance her innate lifesteal even further. Support units are always nice to have.
In terms of ease of build I would say Malice is the easiest. She needs no specific stats at all. Then I would say Shear Force, if you want to inflict Quietus everytime you are going to need max Accuracy. Then it is Don Passione you want Max Accuracy and Max Special Cooldown these two are pretty much mandatory to abuse the Eliza's kit to the max. A good amout of Meter Gain helps as well to get you started asap with Sekhmets.
Unless they want to cause an outrage by nerfing Don Passione/Eliza just after her shard store debut, you should be fine.