If you are unable to reasonably comply with the restriction imposed, then please say so.
There are 2 issues with this. Firstly, the pilot using the R/T often reads back the "clearance" without being able to consider it i.e. the ATCO now thinks the restriction has been accepted.
(S)he now either realises that it is not possible / acceptable / suitable, or more likely, the PF (who really decides if it is) points out it is not possible etc. They will take X seconds to discuss.
Secondly, the R/T is now so busy they cannot get a word in to request / demand / refuse / amend that clearance, and by the time they do, the ATCO has given another aircraft or 2 clearances based on our aircraft's apparently accepted clearance.
I do not know what the solution is? But I do sometimes get the impression ATCOs think we just dial in these numbers (height, speed) and the aircraft just obeys with nil side effects. They do not realise we have *** all fuel, and using speedbrakes, or excessive speed, and then get vectored round the sky / go into the hold makes little sense, and can even be hazardous.
I must add that there are exceptions... and we have been advised of our descent speed / holding duration / descent restrictions 10-20NM prior normal ToD. We can then plan the descent accordingly, request appropriate ToD, and fit in with their speed / altitude / holding profile, whilst keeing things safe, economical & comfortable
