in 2e, its a pretty big difference. A failed stealth check bumps down your stealth status to "Hidden" (What was that noise? Who's footprints are these? etc.), while a crit fail makes you full on "observed" (Tony Tony Chopper style.)

The Assurance: Stealth feat is the lynchpin in many a clunky fighter's exploration kit, since it will pretty much never critically fail, giving them time to hide somewhere or get help from a sneakier friend.