For the lines that I have the stats for - and to be fair, I suspect that people unhappy with theirs were more inclined to supply them than people who were happy, so the results may be skewed:
Rather anecdotally, three lines performed exactly as you'd expect them to for their line length indicating 'perfect copper'.
All of the others underperformed and the common theme was a high noise margin. In some cases the noise margin upstream was much higher, those being the lines with a sync rate of 256kbps upstream, so not even managing the full 1Mbps.
IIRC the noise margin on our line with ADSL was 15db and so at a total length of about 3.7km (2.5 + 1.2) the sync rate was 2Meg with an IP Profile of 1750kbps. Sort the noise margin out and the sync rate might nudge up to roughly the 5Mbps you might expect.
Near adjacent houses had wildly differing sync rates, the biggest variance being 4Mbps sync down in one place and 0.5Mbps down just 300m away. Same cabinet, different "loop".
There is a 1980s estate near us, which might very well be on the same loop as us, and so that may well have aluminium D-sides. I never did investigate further as we upgraded to 3G and got rid of the phone line altogether.
As far as I know there were never any aluminium drop wires, but there were aluminium D-sides. I had attributed the cause of the dismal performance, in part, to those.
That would drop performance hugely, but it wouldn't necessarily cause drop outs. But REIN could potentially impact those noise figures.