I find the game more challenging than the typical open-world game, which don't tend to be difficult at all. On normal difficulty I've died a fair number of times, because I've blundered into situations I hadn't adequately checked out and I've often found myself fleeing from situations that spiraled out of control.
Stealth AI seems to be a bit smarter than the Mr. Magoo AI of most stealth games, with enemies that are easily exploited at rest, dangerously observant in search mode, and tenacious in fight mode. It's easy to kill from stealth, but fairly hard to stay stealthy.
Wading into fights is survivable only because of the rather liberal approach to healing (you can effectively have health kits in triple digits), but even then it's easy to miscalculate and die before you're able to heal yourself.
It might not be as challenging as the FPS of yore, but since it's not a linear shooter, that's only to be expected.