It's not talktalk's network that's the problem - we have about 50 customers all using talktalk's wholesale network with pretty much zero problems that can be attributed to TalkTalk. It's all to do with OpenWretch and their appalling network of corroding copper and aluminium, and their terrible record at actually fixing problems. How your ISP deals with them is what you see, and if TalkTalk retail don't manage them and customer expectations properly (which they don't), they'll appear to be worse than the other ISPs. It doesn't help that their customer service staff are really quite poor in general.
But OpenRetch really do set these companies up for failure. Case in point: we have a client who is reasonably far from the street cabinet and can only get about 30Mbps / 5Mbps service. Every few weeks, the line goes through a phase of going up and down, which is no good for the client because they use VoIP. The fault indicator says there's either a high resistance fault or water ingress to something. Openreach come out, don't find a fault, charge BT wholesale, who charge TalkTalk wholesale, who charge our upstream provide, who charge us. We pay, and then we argue with them six months later for a refund, which we never get.
OpenWretch never fix the actual line problem the right way when they do occasionally find a fault - they just patch the problem. A few weeks later, same thing happens again.
Earlier this month we tried to order a second line for this client. Openreach turn up 4 hours late, start the job, and then run out of time. They won't continue the next day or anything obvious like that - the job needs a) re-appointing and b) paying for a re-appoint (which we need to argue about when we see the invoice and pay).
The whole thing is designed for Openreach's convenience, not the end user. Their job scheduling stinks, and all the customer sees is their provider being bad. Presumably Openreach do a better job if they know it's BT...