I am playing on Casual difficulty, as I just want to have fun rolling around like a bad-ass. I'm finding the side-missions a tad easy though. For example Snatch seems generous with the time it gives me. Are the side missions on Easy as well as the main missions?
The difficulty does not directly effect side-missions. However, things that are effected (how much damage you deal and can take, how smart the AI is, etc.) will have some sort of impact on how easy it will be to complete your mission objective.