I only played on normal...
But yes. Enemies will definitely be tougher, or more plentiful, or both.
About drops... I suppose they will occur far less often, but EXP gained should be the same.
And infestation... does it even need to be faster? I mean, if the Lictor beast kills ya, anyway, the meter will go up.
Say, is there a way to change difficulty mid-campaign?
Because I DO NOT want to start over.
Sadly you can't change the difficulty mid-game. The higher the difficulty, the more enemies you face with higher HP and health. On the plus side since you earn XP with kills you can level-up your squads quicker maybe even allowing you to avoid those monotonous side missions that constantly have you revisit the same maps over and over again.