My understanding is that Gringotts has one specific issue still: vehicle 'check in' with ride control. They're using a newer system that is causing issues. It's a fixable problem, so it's not like prior rumors that vehicles were jumping off the track, 'just' had people riding, and has to cycle xxxx amount of times.
Also, WB does not want guests into the land without everything. They understand ride breakdowns, but will not put up with phased openings.