I've been through several negotiations with various unions, including through ACAS. It really all depends. If the union is doing its job properly and trying to look after the interests of its members without any secondary agenda and management recognise the function and value of the union to the company, you might expect matters to run smoothly.
However, given that other unions have settled and (apparently?) the terms are reasonable, it sounds as if there may be a secondary agenda, somewhere. This could take the form of an individual looking to make a name for themselves, wider union issues that this plays into, external agitators becoming involved, something else entirely. I would be on the lookout for that, as it may not be obvious. Oh, and it could even be on the management side.