I used 2.5-3" gold screws ( stronger and a step above drywall screws ) on the holes that I was drilling into studs behind the stucco.
( only needed to do this on 1-2 of the junction boxes holes .. the other side I used a shorter screw into the plywood / wood board )
I pre-drilled a hole into the stucco with a masonry bit until I hit the wood, then pre-drilled a bit with a normal bit to the depth that I needed.
Worked well.