I don't feel any of the enemies look bullet spongy from the footage shown. They drop fairly quick. Even the bosses with a squad of 4 I imagine drop in seconds when coordinated well.
Seeing event footage without optimized load outs, levels, coordination, etc isn't too reflective of what we'll experience once we are playing it in the beta.
Plus, bosses should absolutely take time to take down. Hence why they are bosses.
I don't want it Destiny style of say the tank before the big eye boss fight where he just takes forever due to way too big of a health bar so you waste 10-15 mins on a mini boss. I do want it to be appropriate though that if I'm fighting say, The Oryx of this game, then yeah this big bad ass should take 15+ mins to take down if done right because of different stages of the encounter, etc.