As a long-time Halo fan, I'm quite disappointed in Halo 5, and honestly I could probably fill a book with reasons why I feel the game is pretty bad. One of those main reasons is how awkward the aiming system is, but it's not just me--gamers have proven that Halo 5's targeting is legitimately unbalanced.
A Redditor by the name of Z0oinks has proven that Halo 5's targeting is bugged with a simple video that shows the issue in action.
What's happening is that Halo 5's aiming system is faster for linear X and Y axis aiming than it is for diagonal 45-degree aiming, creating a very clear disadvantage across the board. The awkward aiming becomes obvious when the user moves in a full circle: instead of being smooth like other Halo games, moving the reticle in a circular motion is broken and jagged, meaning users will have to compensate quite a bit.
Since the Reddit thread hit the front page, a 343i rep jumped in to say that he's passing it on to the "right people" at the studio and that they'll be looking closely into the problem.
"You should rest assured that anything that makes the front page (or first few pages, really) is not only looked at and fully reviewed, but immediately discussed with game and services teams who are rotating on 24/7 shifts. That said, many of you have likely seen Ghost's reply in the other thread, but we're also passing this on to the right folks to ensure that everything is working correctly."
Aiming is the most important thing in any shooter. It has to be refined, balanced and perfected. You can't have any facet of the aiming and targeting system being affected in this way, or it'll unravel the magic and fluidity that is a first-person shooter.
343 Industries used help from Halo champions while developing the game, allowing these gamers to test and deliver feedback to help shape the game further. The problem is that these players have played the game a ton over a two-year period, getting used to the skewed aiming system so they didn't really notice what was happening.
Even still this kind of thing should have never passed through testing and certainly shouldn't have been shipped. In actuality, there are many parts of Halo 5 that make the game feel unfinished. While the game continues to get its praise, many gamers are quite disappointed with the game and have written off Halo altogether.