I've played one match of Arena CTF and a few Warzone games. Is incredibly fun. They've somehow changed a lot of stuff yet it still plays like the Halo I remember, which I haven't really played much at all since Halo 2.
I can see reviewers scoring it in the mid 8's. The online is going to be ever growing, ever changing. It's like Titanfall or Destiny. The scores it gets now don't really represent the quality of what the game might be to you now or others in the future. Some put more weight on the SP. Or all weight on the SP. Others more MP or all MP.
You see a lot of trolls talking about how Halo has fallen and 343 can't match Bungie... well their first crack this gen has so far matched Bungie's major revision at trying to unfuck all the things they fucked up in Destiny. And all shooters as a whole seem to be reviewing much lower this gen. I don't think any of us have doubts that had hyped shooters like Titanfall, CoD2K14, Destiny, and Halo 5 launched last gen they'd have all scored 90+ easily.