Sometimes it's conflicting with regards to the darkseekers...The one he catches in the bag was thrashing around like a maniac and 1 hit from the butt of the gun knocked it out. On the other hand two of them were able to smash into the side of Neville's SUV and tip tip it over on to its roof (although it's driving up a ramp on 1 wheel which made it more vulnerable to tipping at the time) although I think they might have either badly hurt or killed themselves in the process.
I think they have it in the speed dept as well which may have contributed more to the tipping over of the SUV that sheer strength with just the kinetic energy tranferred in how fast they run.
The reapers are able to kill vampires and they have genuinely higher strength that humans hence I think 1 on 1 they would just tear the darkseekers apart but when the darkseekers swarm and just run at and eventually collide with a reaper then they would all dive on the reaper like they did with Neville's car and overwhelm it probably sacrificing a fair few of themselves in the process.