In the good ol' days the flow was the only one doing the times - he'd do all the fiddling of figures and we'd just pass it on. Things were set in concrete early. Now there are many fingers in the pie and the times aren't set in concrete until much later - the dilemma for controllers is give a time too early and it's likely to change several times, frustrating and time wasting for everyone.
Maestro is another thing for us to monitor so if we're busy separating traffic we might not look at it for a bit and not notice you've gone from no delay to 6 minutes - we don't get directed warnings, it just happens.
We know you like times early and I'll try to slow you early when I can but that relies on me even noticing you before you're handed off to me - when it's busy that often gets missed because it's low priority.
Re. efficiency of constant profile descents and such - I cack myself laughing every time I read of the latest scheme proposed to save fuel. You can achieve whatever you want when you're the only aircraft in the sky but as you say, the more aircraft the less likely that is to happen.