I have not played DF before, only the base game, but I am also under the impression that it is much more difficult. Either that, or I suck at this game.
In the very first run I had to restart completely because I had used up so many medkits that I could not survive the 7 rounds in the basement. Second run was also harder than I thought. I am barely able to hit anything, percentages usually are about 20% while my own team members constantly get hit with criticals. I also don't really understand the flanking mechanic, it sometimes seems strange what counts as flanking and what doesn't.
To me the fights are still kinda easy, harder than it was before but not impossible. The flanking mechanics, on my experience, work this way:
The target is on front of your selected character and is behind heavy/medium cover, the target gets cover bonus and best of cases you are only going to do normal damage and, most of time, half damage and with enemies having armor, your damage will be low.
Cover only offer you protection if it is exacty between you and the enemy that is shooting, if the enemy is on your right or left (it doesn't need to be totally to your right or left) and the enemy can shoot you without the bullet passing through cover to get you, you are now being flanked by that particular enemy and are going to receive criticals like crazy from him with damage ranging from normal to double. If you hide behind a single box (doesn't matter if it is light, medium or hard cover.), it is extremely easy to be flanked as the enemy just need to move a bit to the left or right to shoot directly at you, ignoring the box.
If your character is stunned, prepare to be butt raped by flank shots as he is considerated out of cover.
Any character out of cover is automaticaly on flanked state and has a big chance of being raped by flank shots by everyone nearby.
Because a character is on flanked state, it doesn't mean all attacks are going to be flank attacks (but with high weapon skills is almost guarantee you gona rape an enemy that is out of cover) but there is a big chance.
You don't need to be exactly glued to cover to be protected by it, if the bullet has to pass through cover to get you, you won't be flanked.