From a ‘bus perspective, all we can do is adjust cruise and descent speeds to make the crossing time, and its usually quite a ‘bastardised’ process
The bus actually has an RTA function which works quite well in my opinion. (Within 30 seconds, and designed to keep you within 30 seconds I think).
However you often have to tweak it during descent if you really want to meet the exact time.
This is not a system flaw but perhaps just because the descent winds might be different to forecast.
Also once on descent, the FMS gives priority to maintaining descent profile compared to meeting a fix at a certain time.
I have flown with old school pilots who openly crunch numbers on the way down (ground speeds, time and distance, miles per minute etc ..without a calculator or a whizz wheel....geeze they really are so smart) as they think out aloud (or to show me how smart they can be be without the FMS) but it really is not necessary.
The people who say it (the RTA function) doesn't work just don't know how to use it.
Fair enough, whatever works best for you.