On the A320 with the A/P engaged the speed brakes will only extend to the half way position, even if the lever is pulled back to full. If the A/P is then disconnected the boards go to the full position. This causes a huge increase in VLS (lowest selectable speed). If AoA protection kicks in brakes retract.
Why the QRH emergency descent procedure calls for full speed brake selection when with the A/P engaged only half will deploy anyway, and you will be caught out if the A/P disengages is a bit puzzling ?
In the event of an emergency descent the technique is to first allow the speed to increase before deploying the brakes. At high altitude and high weight with a 30kt coffin corner I would not want the brakes out unless I was descending and very closely monitoring the speed bands.
A delayed descent in managed mode will have the speed target uncomfortably close to the barbers pole, especially in turbulence. Also if speed brakes are used in managed descent, as soon as the aircraft thinks it is below the profile the engines will power up against the brakes (you do get a warning though).