Its quite frustrating when in Purge missions, you have to backtrack a large section of the map because you missed a few mobs (more frequently occurs on the large open maps). This is especially problematic with the Psyker, as you can miss demons summoned by the high warp heat. Either:
1. Add a tolerance to Purge missions; make it so that you complete the objective by killing around 90% of the enemies on the map (when Caius Thorn comments on you getting near to the goal).

2. Make it so, when Caius Thorn comments on you getting near to the goal (remaining enemies are revealed), all remaining enemies start to converge on you with high speed/teleport near you, so you don't have to backtrack to kill them.

This quality of life improvement would increase the viability of Purge missions (imho they are quite bad, as this issue and the nature of the mission itself makes them slow compared to a Hunt for example).