Note that I do not have a training manual for any crafts yet, but these observations are about the differences between synths that suggest manuals vs synths that don't. Not about the difference between manual / no manual on a synth that suggests it.
I've always felt like Nails / fish hooks were really fucking hard to synth for some reason, even when I'm over their level. Yesterday I noticed that it's not necessarily because my Standard attempts succeed less often, but because when they fail I'm losing 25-30 durability almost every single time. Take the following example:
Was skilling up yesterday as a 19BSM. Started out with Iron Fish Hooks since it didn't use Wind Shards, seemed like a win.
Fail Rate = ~50%
EXP on Success = ~850
Skill on Success = 170-210
The fail rate obviously does not match the EXP on Success. I switched to Iron Needles, and the resulst were more like this:
Fail Rate = ~25%
EXP on Success = ~1300
Skill on Success = 280-330
Something is clearly off here. I've definitely noticed that the amount of durability loss you take on an unsuccessful attempt decreases the closer you get to an object's level. So if you try to synth an item like 10 levels away you're going to take massive durability loss on each failed attempt, and that durability loss will gradually decrease the closer you get to level.
The training manual seems to do the same thing. When I'd fail my Standard attempts on Fish Hooks (which are lower level than Needles), I'd lose 25+ durability a lot of times. On Needles, durability loss / failed attempt was more like 10-15.
This isn't like a groundbreaking discovery or anything, but I'm wondering if the same applies for other synths with training manuals that people have observed.