I still haven't made it through the campaign. I can't even point to any specific flaw with the game (gameplay feels perfectly fine).
I just don't care, which is probably the most chilling thing I could feel about a Halo game.
I've been that way a couple times over the past 15 years. The first time was Halo 3. When it came out I wasn't really too excited (I don't know why because I loved Halo 2 and had been wanting to see what happened next), and once I started playing I just found it a chore to get through because I found the story and gameplay to be a boring rehash. I played it once then shelved it. For some reason I still bought ODST and really wasn't feeling that one, I played it a couple of hours then never finished it until I got the Master Chief Collection. My interest was at an all-time low until Reach came out and I started to love the franchise again. I played it so much though that by the time 4 came out I was kind of burned out on Halo but I still picked up a copy just to see what was up with the Chief and shelved it after one play though.
I bought Guardians alongside the MCC and Star Wars Battlefront when I picked up my X1, but I was really more interested in the latter. Though I shouldn't have been, it's kind of a mess in my opinion. I had a little fun with Battlefront in the beginning though, and I basically just played through Guardians once to see the story and then didn't mess with it for a month or so. I thought it was OK but I wasn't completely feeling it, at least not enough to play it over and over. But once I was just about fed up with Battlefront I busted it out again and have been playing it quite a bit, mostly the campaign. I think it just took me a couple of play throughs to really get used to. I just finished a Legendary/Lone Wolf play through today, which I really don't recommend. At least not expecting a good time, just if you want the achievements or to see if you can do it. It's probably the easiest Halo game to beat on Legendary (it's more a test of patience than skill) but it's also one of the more irritating. I think they should rename it Bulls#!t mode.