Completed it today on Hard, not the longest game in the World but just about long enough.
I concentrated on the skill tree that gives bonus XP first to maximise the levelling potential, took a while to get around to combat upgrades but once you increase your engineering skill, which increases the AUTO-9 upgrade percentages and also the combat skill tree with 50% damage bonus that gun is insane.
Didn't help with my first
as it meant I was a bit underpowered, but subsequently I felt pretty OP for the last 1/3rd of the game and all the big battles got a lot easier.
3/5 from me. Good while it lasted but won't play it again. Could have done with a better story/script but the shooter mechanics were fine. They do sort of introduce new enemies as you go through the game, like a guy that calls extra reinforcements, and another that buffs the enemies on-screen, but they are few and far between.