I think it depends who you get. The people populating their online forums are good, and I got good service out of their house-move team.
If you have a complex problem, where continuity of the support agent is important, then they can be a little hit-&-miss. Getting on the forum (and approaching the problem with an open, willing mind) is the best approach.
Problems do happen, including with fibre. My experience, and what I have read on the forum, is that a lot is caused by "their suppliers" (or their suppliers' systems) - generally a euphemism for either BT Wholesale or BT Openreach. Understanding this, and having a little patience while they attempt to bring "their suppliers" into line, seems to be key to getting reasonable service.
Of course, not every agent is a "superman", so things do go wrong there too. Again, feedback on the forum seems to result in the information being taken on board, and passed on internally.