There is very little disadvantage to setting it to 90. Proper field of view relies a lot upon size of your viewing area (TV, monitor) as well as distance between your eyes and the screen. Personal preference also factors in greatly. 65 is often appropriate for sitting on a couch some distance away from a TV, while it is generally far too narrow for sitting on a chair in front of a monitor on a desk.
As kotekzot points out in the comments, higher FOV can make aiming harder, as your enemies now effectively take up a smaller portion of your screen.
A too-narrow FOV can cause nausea for some players, while a too-wide FOV can look very fisheyed and distorted. A wider FOV also requires more of the game world to be rendered and can result in worse performance (relevant for console hardware and lower-end computers), but also has the obvious upside of showing the player enemies coming in from more angles.
If your hardware can handle it, I would set my FOV as high as the game allows without causing so much distortion it bothers you -- most games are restricted far below the 110-120 degree upper limit for computer gaming becoming too distorted for most people, although certainly some people have lower tolerances.
So higher FOV does give an advantage in awareness, although there are a couple minor drawbacks in distortion and potential hardware limitations as well as making aiming slightly more difficult. Lower FOV does not cause distortion but can cause nausea and motion sickness, while being easier on hardware. Pick what suits you most, if you are lucky enough to be playing a game that doesn't force a low FOV on you.