But lets say you're at 220kts...you'd want 1100fpm for 300ft/nm descent. Idle power, in the A320 will typically give around 1300fpm-1500fpm (apologies if you're already well aware!) Admit it's not much power though to reduce by 200fpm-400fpm.
Not sure about other bus drivers but I find the vertical management is pretty useless below 10,000ft when wanting to achieve a CDA, given it programmes a level flight portion to slow down. Some work mates swear by it though and prefer to get down to about 4000ft and choose V/S 300fpm in the last 1000ft before G/S intercept....does this doesn't qualify as a CDA?