Hello - my first post, although I have been reading this forum on and off for some years. I am some sort of a has-been weekend pilot (i.e. you don't want to fly with me
), with some history in design and standards related to communications avionics.
To the topic: One aspect I am aware of is that Europe has been severely constrained in terms of VHF frequencies for datalink (VDL mode 2 and ATN). Only a very small number of frequencies are available for the whole continent, and are reused over and over. Some work was done to relieve this, but the last info I have is that it remains a bottleneck. This problem resulted in a program stretching over more than a decade which resulted in adopting satcom as a means to augment VHF datalink (ATN) over Europe. The first airliners started using this perhpas a year ago, with the number of installed aircraft now significantly ramping up.
I can't say that the VHF congestion explains everything the OP reported, but it is certainly a factor. I imagine it would vary from area to area.