I used scientific method in reaching this conclusion. Anyone can replicate my results.
1. Go to the outrageously outdated wiki: http://pillarsofeternity.gamepedia.com/Status_Effects
2. Press "CTRL-F" and type "reflex"
3. Press the "Highlight All" button in the bottom left corner of Firefox window.
You will note that there are many more Reflex debuffs than Will and Fortitude ones. Fortitude and WIll ones tend to come on the same effects - Charmed, Sickened, Weakened.
Sagani can use Wounding Arrow to inflict Hobbled, which reduces Reflex and makes the target susceptible to Binding Roots. She can also use Stunning Arrows. Hiravius can use Sunbeam, which blinds and further reduces Reflex.
There ARE some spells which target mental stats in a sneaky way, for example Miasma of Dullmindness simply drains -10 from three stats.