I'd be surprised if it didn't encounter delays of some sort. But you'd hope the programme is realistic and allows for them. We're fixed in when we can do certain works, because of needing to access the pitch and have no fans around. So they'll have gone through those timings with a fine tooth-comb to ensure it's deliverable. But bad weather, delays to materials or suppliers are all common place and cause issues. When you have so many moving parts, and a lot of temporary work to facilitate the construction, plus a complex roof system than needs holding up - it's one hell of a job to manage it and ensure it happens.
I haven't seen whether they're currently on schedule or not. Often it's the groundworks phase that causes delays. Encountering something you weren't expecting or weather that causes a problem. Once that's done, often it shoots up without an issue.
I haven't followed the arena closely enough to know why it's been delayed so significantly.