Craftsmanship and magic craftsmanship are not really hidden stats. I don't know if SE thought they were just that obvious, or if they had other things to deal with.
Craftsmanship controls your chance of HQ Quantity (aka you make 16, 24, or 32 instead of 12)
Magic Craftsmanship controls your chance of HQ Quality (aka you make a +1, +2, or +3).
Magic Craftsmanship seems to help off-hand more than main hand (most of the synths I have seen that do +1s are off hand... not all).
That's not entirely true.
There are some synths where main hand HQ results are +1/+2/+3 and offhand are higher quantity. And since we know that rank for rank, main hand tools tend to focus more on craftsmanship and offhand more on magic craftsmanship, trying to offer up concrete explanations for how things work without recognizing the bigger picture is a mistake. The hint comes form the types of tools. Pliers are delicious for making rings out of wire for armorer. They seem to be better for making chain out of rings, too. A spinning wheel seems better suited to making yarn than a sewing needle. A grinding wheel seems better suited to polishing gems than a hammer. A mortar seems better suited to grinding flour and other powders than an alembic.
The way to get an idea for which recipes favor which types of tools is to look at the HQ results on a site like YG that now splits them between MH and OH for a lot of recipes. As a general
rule of thumb, the tool that produces materials of a greater quantity
on an HQ is the hint over which stat the recipe favors.