Engineer felt useless in POC.
In Nightfall, you really want a good engineer. Mainly for keeping electric traps on at all times in third area. If you do this the 3rd area goes from hectic, to almost easy.
Every game with traps is 500 to 600+ kills.
You will max out money( @$8000) often enough to drop money for your team frequently, even while dropping ammo or team support.
I usually level up my portable turret first (for the breeder) then engineer, then feral instinct. Plus whatever else you can manage.