Samara, Reave, and Lokis!

Samara's Area Reave is not strong enough to full strip armor off the most basic enemies at level 30, even with all damage upgrades. Samara does not have enough native power damage or duration to compensate for the -20% duration penalty incurred on Insanity. Even the husks on the Derelict Reaper, she can *appear* to strip them with Area Reave, but there will be a magic pixel of armor left.

Insanity has a -20% duration penalty to all duration-based powers. Hardcore has a -10% duration penalty. (Maybe I should say powers with a non-fixed duration. Inferno Grenade for example has a fixed duration for its damage over time, as does Incinerate. These durations do not receive the penalty as far as I am aware.)
I do not know if Veteran has penalty; I do not know if Easy or Casual has a bonus.

Reave does damage over time when it hits health. When it hits shields/armor/barriers, it deals its damage all at once. (It deals 2x damage to armor and barriers). Total damage = damage per second * duration. But since the duration penalty affects the duration, it affects the total damage.

NOTE that the duration penalty affects the base duration. I have not tested it, but I believe (or recall?) that the duration penalty changes the base duration; this means the -20% duration penalty and +20% Biotic Duration upgrade DO NOT cancel each other out. So the power duration calculation would be like: (Base duration - duration penalty) + sum of duration bonuses.

For example:
(4 - 20%) + 20%
= (4 - 0.8) + 20%
= 3.2 + 20%
= 3.2 + 0.64
= 3.84 seconds

Sure, I remember this.  But this is because as you leveled up, her area reave became less effective due to increasing enemy strength.  At lower levels (23) she used to be able to fully strip Lokis with the area evolution and Sapiens Justiciar.  Examples: 

youtu.be/-gJGMhYCOtw?list=PLLwb5P89imByPOP7tym8WPORX9YOoy7la&t=97
youtu.be/-gJGMhYCOtw?list=PLLwb5P89imByPOP7tym8WPORX9YOoy7la&t=150

I've been playing this mission again at levels 27 and 30 (6/6 biotic damage).  At level 30, area reave does exactly as you describe - strips all but an invisible sliver of armor.  But at level 27 it leaves behind more (not less) visible armor.  In other words, as you level up, you strip off more and more armor (not less and less as used to be the case).  So it seems they have changed something about how enemy strength scales with player level.  Since the end point seems to be the same at level 30, I surmise they must have made enemies stronger at lower levels.  Maybe this is so you always feel like you are progressing rather than regressing in damage as you level up.  

The practical implication is that Samara is no longer a no-brainer inclusion.  If you want to area strip armor you have to take Mordin, and thus lose 2nd Puller.  Or you can take Samara specced for Heavy Reave + Pull, but must give up AOE armor stripping.  I think Mordin might a stronger option for Adept/Vanguard in many situations.

Sure. This is the exact reason I has Samara on nearly every mission. She could pull (with lightning fast cooldown) or AOE strip to set up cryo, slam, pull, and instant Pull-Warp combos. That situational flexibility was golden (below level 30 anyway). Now it's more of a real tradeoff, which is arguably better balanced.

Don't get me wrong - she's still great with Heavy Reave, but you can't spread utter destruction quite as well, since surrounding enemies still have their armor or barrier when the Warp detonation goes off.