Although CPDLC has indeed still some way to go. We have seen considerable improvements lately at EDYY (MUAC) and there are more improvements on the way for the end of the year.
However so small at times, it can indeed help with the workload on the sector.
I know it can be frustrating for a pilot to log-on to EDYY and only get voice messages.
There can be many reasons for this. But overall a logged-on aircraft can help out the workload of the controller.
Mind you as well that there are always at least two controllers working any sector. So if the R/T is busy and you have a request feel free to do so via CPDLC. Chances are the Coordinating Controller (or Assistant Controller) can pick up on your request and incorporate it in the overall plan for your flight. If this is unable or ATC wants you to stand by, in both cases these responses will have be sent by the CC(AC). Again reducing the workload of the EC.
The Executive Controller will choose the best means for him/her at that time to rely you the reply.
The controller can see if an aircraft is equipped but not logged on and therefor you may be asked to log on. However to question all equipped and not logged on would increase the workload. Hence the request in the NOTAM.
Feel free to always try and log on to EDYY when with "Maastricht Radar".
There is a difference in performance depending on the type of CPDLC you have but as well the combination of hardware and software. These should normally be known to your company.