No, it isn't a downgrade in FOV or display area.
You just get the sides extended.
Example
I have seen this posted a couple times in this forum now, and you aren't losing any vertical FOV at all and you get increased horizontal. That is the entire point of 21:9 vs 16:9.
Another example.
Unless a game doesn't support a 21:9 resolution (typically 3440x1440), the image isn't streteched or distorted. You just seem more horizontally.
EDIT: Unless you are comparing it directly to Triple Monitors, of course :) Then yes, Ultrawide isn't as wide.