Big potential ruined, what a pity for such an unique setting.
First, shooting is fun. Really, I don't see why it was marked as bad, it feel great and its one of best made things in the game. Sound of the rifle is right, not like games where it feel like you're shooting from a toy-gun. It have deep immerse sound of the rifle. Other weapons are fine too.
I like setting of the game - Indians, American civil war, ww1 etc.. its great and new.
But:
Your character is silent protagonist. You're fighting in your time, only when suddenly escorted through time/worm hole to... cca. 1 room with 1 screen and 1 character. You'll get a brief info whats going on and start missions by selecting which one you choose(but must still complete all of them to progress anyway). Now this was silly, for company that research time travel I expect full area of labs and lot of people working there - half-life 1 anyone? It felt so limited. Another big letdown was that higher-up female called "mother" who give you orders. Her voice is so irritating it annoyed me to no end. You see some ugly eyes on the screen and !*so*! annoying voice coming from it you wish briefings would be just text based. Your companion is tard as well, didn't like him. Sure, you can say this isn't stopper, but for me how it was presented completely ruined belief and whole story.
It still could be ok if gameplay was excellent, but its a mixed bag. First, AI is bad because it behave very erratic. Second, enemy spawn badly, even in the middle of group of allies. I often felt enemy is spawning in places where I was sure it have no way to come from, like behind me, behind rock etc. that I was 100% sure to be clear.
Now to the maps and level design. Its more linear than it looks as there are invisible walls but this wouldn't be such problem, its that this game was designed like big area where you could(and should be able) theoretically go anywhere. Levels are big areas on the map. In linear shooters there is always hint that make it clear you can't go out of rail, like steep mountain or some blockade. But maps are here designed similar way like Oblivion or Chrome and so you get surprised when suddenly can't move further on flat terrain just because you're not supposed to. I believe it is because most of objectives are about to reach some point on map and they wanted you to go specific way. And it could have been such a great potential to... for example infiltrate enemy camp where you could choose any tactic from any strategic position on the map, if only you were free to explore.
So what we get is unique shooter with great potential ruined, great shooting feel and great unique setting but unfinished product. All that drowned by erratic AI, ruined storytelling, few badly designed gameplay mechanics and presentation. And keeping history intact made game less exciting I guess + preserving certain characters alive annoying. Would not be great to change history? Imagine some epic levels where you assassinate Hitler himself, save or even kill Jesus before he get famous(as many people in history died because of stupid religion) or prevent nuclear strike on Japan in WW2! Just quick examples from my thoughts, those are not here...