I've never used it for knife scales but I have a mallet made of lignum vitae and several vintage small round rope guides (the kind used in sailing) and have semi-abused them and the rope guides don't even show appreciable wear after years of use outdoors left exposed in all kinds of weather. It was also used as a bearing/seal on boat/ship drive shafts where they exit to the propeller. Tough stuff.
It's not typical for this wood to fail. I wouldn't be too concerned about the scales pictured failing, they look solid.