There's been the odd exception but ultimately it got resolved.
Like January to May 2012, and January to April 2013? (I forget exactly how long, but months in each case).
If you're unlucky enough to be in a congested area, then it's hell and remains so for month after month. This experience also provides me the experience to say that Sky 1st line support is bad
, really really bad. I can't speak for BT but Zen is 1e100 times better than Sky.
I have both Sky and BT Infinity unlimited fibre from the same cabinet. Currently speeds are similar on both but pings on BT are 6ms while Sky are 30ms - was 20ms, then 1.5 months in I rebooted the router (not modem!) and it increased.
That said, I had a Zen connection on the same exchange some years ago and congestion was awful. At a university town, and literally the day students arrived it'd drop to 300kbits/s from 8Mbit/s, 9am to 2am. That lasted for over a year. It affected everyone non-LLU no matter the provider, so was BT's problem, which is why I went with Sky LLU.
And having said that
, I've had ADSL at another address for 10 years (last 8 years with Zen, current sync 17Mbit) which has never, ever shown any congestion issues. Full speed, asymptotically flat out all day every day with fast ping (<20ms, which seems good for ADSL).
I'm not sure what my point is. But Sky wind me up something rotten.