I'm with KEG, RTA works well as long as it is used correctly and understood. It won't work if you stay at cruise altitude and min speed won't achieve the time, how can it? Adjusting your descent speed before entering the RTA will make a difference.
When I'm given a time to achieve I reduce the cost index to achieve a descent speed of 250. I enter an RTA. It should be obvious whether you will be able to achieve the RTA at your cruise altitude. As a rough rule in the aircraft I fly with 150nm to run to the fix for every min I need to loose I will need to descend 2000ft. I.e.. If I'm at 37000 feet and need to loose 8 mins I will descend to 21000 feet. The descent need to be a normal (flight idle) descent, none of this mucking around at 500 vs. the faster you get to the lower level the sooner your TAS / GS reduces. Then RTA will do the job very nicely thank you. (Within 5 seconds is the norm)
You other choice is to remain at cruise level and hold, burning more fuel.
The difference in fuel burn between 37000 and 20000 is around 300kgs/hr. Not much but over several flights it adds up. It also seems to help ATC as they regularly voice their appreciation for our efforts.