The problem they had was that when customers had multiple accounts with the bank of different types. You would have diminished the customer experience. And migrating some customers entirely demands complete functionality on all products.
From what I hear Lloyds were also part of the problem. TSB were tied to them, paying an arm and a leg for a static service and any phased approach (e.g. Migrate credit cards first but still show all the products on the Internet Banking) would have needed unbelievably expensive services from Lloyds. So TSB were screwed. Can't get Lloyds to help you (because Lloyds have no interest in making it a success), so you have to make sure it is going to work in one go.
I think the failure was when TSB was bought from Lloyds. At that point, the few people around the world with experience of massive separation of banks (and it will only be a few experts) should have crawled over all the commitments and costs Sabadell were getting Lloyds to commit to. Lloyds gave Sabadell £400m to subsidise the separation (I think that's the figure). That is far too small and actually, much more valuable than money would have been a committed capacity of Lloyds IT support staff during the separation, without the horrible management layers to get anything done between the companies.
Major problems were that all the IT knowledge was in Lloyds, TSB were just users. So when Sabadell asked TSB 'now tell us what you need your systems to do.?' they just stared at the techies and said 'we don't know, we just use the systems, we don't know what they actually do or what they do underneath' and Lloyds wouldn't tell them either. So Sabadell had to reinvent everything just to stand still.
In an ideal world you would parallel run and prove the systems 'green field' with new customers, then migrate portfolios of existing customers onto systems that have already been proven with new customers at low volumes.
But all of these approaches demand proper support from Lloyds, who had no interest in making it a success.