>>1923458
Lack of documentation sounds about right, about crests: You'll get powerful crests later on, the ones you'll get in early game are not very good, Slayers crests you can pretty much ignore and 14 is not actually a percentage but it's added to your base damage, it might not seem significant but these type of damage boosts come before any multipliers from skills and stuff so they can be impactful, that being said the Slayer effect from crests is too small to be worth it, once you try BBM you can get bracelets that give you Slayer effects that can go up to 90 and those are more useful.

About augments not being very descriptive, I assume that's just how they were originally in Japanese, not very detailed, but if you know Japanese you can try the Japanese version of the game and check for yourself.

The latest updates to the translation have started adding things such as numbers like percentages and such to many augments, it seems like you don't have it installed, do you happen to have the new launcher? It will let you quickly update the game translation with this button, that button also allows you to set the game to Japanese if you want to.