Managed descent with selected speed, another untidy thing except if you are high on profile at a high speed. When on profile with a selected speed similar to ECON speed, SPEED and THR IDLE modes will alternate annoyingly.
If you select a low speed, you will get high. If you select a high speed, SPEED mode will keep the high speed an pitch will keep the profile, but... If speed is much higher, airplane energy is much higher too, and if you don't pull for an OP DES early enough, in the end you will have to use speedbrakes! That's another classic, by the way. I have seen it happen many times, as follows:
ATC clears us to a mid flight level, such as FL240, well away from arrival airport. Then pilot selects FL240 as the cruising FL, so you have a TOD in the MCDU. Pooblem is that ECON speeds will change and ECON descent speed may be quite slow, like 265 or something. So when reaching the TOD, the pilot pushes for DES and when realising speed is too slow he selects, say 320 knots (he wants to arrive early for supper).
Pitch will maintain the magenta profile, which is calculated for a 265 knots idle descent. SPEED mode increases thrust very noticeably to maintain that shallow path at 320 knots. Pilot requests "high speed below ten". Everything seems alright. Then there is a moment where the pilot, being on profile at, say, 20 miles, starts deceleration and then something smells rotten and the pilot deploys speedbrakes. Finally he manages to do the ILS only it took early configuration and speedbrakes all the way. Why? Because he had about 70 extra knots of energy. The profile was never good. He was never on profile. He has been using thrust all throughout the descent and at the end he had to use speedbrakes and early flaps, maybe even early gear down depending on the particular circumstances of the day.
These mistakes stem from two added factors:
incomplete knowledge of the airplane's managed automation plus lack of knowledge of the airplane performance and behaviour without the managed modes and without automation at all.
Both are unacceptable. Pilots must master both skills: managed and selected automation and manual flight.