The reviewers for this game play too many high end games to appreciate this one. Darkest Of Days is very underrated.
The critic who reviewed the game does even understand the "aura" soldiers. The reason you can't kill them and other soldiers can. Is because you are not from that time era and everyone else is. Also, it's not like you get punished. Kill the "Aura" soldiers and when the portals open up, kill the opposition agents and take their kick ass weaponry if you think the game is too slow.
In every level it is possible to get a futuristic weapon. Whether if Dexter gives it to you, or if you kill opposition agents. By doing this, it makes the game go by faster if you don't like making the game feel more real by using the weapons of that era.
It's always funny to hear that some people feel that games are racist. Of course you are going to be shooting a lot of confederates.. You're in the UNION army trying to find this guy to bring him to safety. If you were looking for a Union soldier, would you dress up and fight on the confederate side? No. Even though you kill a lot of Confederates, you do kill a lot of Union soldiers too. Your character does not choose sides in this game..
All in all, it is a fun game, it's innovative, it's one of a kind and you should really check it out if possible.