My suspicion is that Gara does not do the kind/type of damage that she is supposed to do with Mass Vitrify and that this problem translates into a problem with Splinter Storm.
Problem: Gara should be able to determine the damage type that ensues from destroying the Mass Vitrify barrier by using either Thrust (puncture) or Sweep (slash), but irrespective of choice the damage numbers are always the same, provided that the amount of damage done by Thrust/Sweep is the same.
Simplified example: An enemy that is weak to slash but not puncture damage should take more damage from Sweep than Thrust, but this is not so.
This problem translates into a problem with Splinter Storm, since Splinter Storm should absorb the damage type done by Thrust/Sweep. At the moment, an enemy damaged by Splinter Storm takes the same amount of damage whether the barrier was destroyed by Thrust or by Sweep, provided that the same amount of damage was absorbed from Thrust/Sweep by Splinter Storm.
Furthermore, it is unclear whether any elemental damage is done at all by Mass Vitrify. The alleged 800 base Heat damage from Mass Vitrify has not been seen a single time in testing.
If correct, this erratic behaviour makes a mockery of an important aspect of Gara’s design (that she is supposed to be able to tailor her damage output depending on the type of enemy she is up against) and detracts from her overall viability.
Further testing and examples in the video below (at roughly 56 minutes):