Though I've been owning this game nearly from the release, just managed to finish this few months ago.
In my opinion the first half of the game is more puzzle-like where you do the hardest thinking. Last half is more like blasting your way through, albeit often with a serious hurry like in the whole game.
The game is fairly challenging in the beginning missions when you don't have much ammo or melee options in your assaulters/sewer guards. Then it gets a bit easier untill around after the robot levels and gets harder and harder all the way through the factory assault and when you start to encounter spiders.
The end game wasn't as hard as it could be - I had acquired experience on all my guys and hadn't used commendations, so when jetpacks came into play I equipped my team with them and after that it went rather easy. I'm not sure if the game was balaced around that.
Tip: Trying to examine all chests in every level results in usually MUCH harder runs, but meh, reward is a reward.
Tip: Don't conserve on commendations too much. I had plenty of them left in the last mission, so it seemed like a waste to save them.