Disclosure: I previously worked for TT, but not in Broadband.
Many of their issues come from the fact that they are a mismatch of different companies which were absorbed and integration of those different businesses over the years has created massive legacy. Furthermore a more significant part of their issues with provisioning and maintenance comes down to OpenReach and the way they are integrated with them. The installation of my staff incentive phone line was a farce, but it all started to go wrong with an OpenReach engineer failing to check the right boxes, then rectifying that clerical error took too long and was too hard.
TT had invested substantially in their core network and its capacity was astounding, they had the fastest DNS cluster for customers in the market and the fibre core capacity was good. One area they struggled with was again BT, they are largely dependent on BT and even when they have their own kit in exchanges they often need to use BT's fibre to get to the exchanges. Some BT exchanges are just child exchanges of larger exchanges and so they only have limited data capacity upstream. That being said TT's ADSL2 kit was ancient and was desperately in need of retirement.