Backliners get quality cover on every map. While some maps give terrible cover to front liners (Atrium) or mid liners (Terminal), backliners will always have cover that is sturdy and large.
Backliners also have the smallest hitboxes. It seems really unnecessary given that they are already farther away, but HHG doubled down and made them small enough that they become much harder to hit, especially when moving.
Backliners also gets generous HP allocations. While they never reach the same amount as the tankiest of tanks, they are still able to match or even surpass the HP of even “tanky” characters. For example my gold 8* Nightingale gets 38.84hp per power, but my gold 8* Hivemind only gets 37.06hp per power. My Francois gets 37.85hp per power, my Steele gets 39.43hp per power, my Dogface gets 34.11hp per power, my Bolt gets 25.81hp per power. You’ll find far more squishy midliners than backliners for some bizarre reason.
What about armor? Well… what about it? Armor is almost irrelevant right now. When HHG made armor less punishing against high ROF weapons last month, they also made it literally useless against low ROF weapons in the same stroke. Now a tank shooting Nightingale might only lose 20-30% of their damage to her armor, but there’s almost no tank that I’m aware of that will block ANY of Nightingale’s damage.
With shotguns now having serious falloff against backliners, I feel like this PVP meta is only becoming more and more biased toward backline characters.