5,000 is a critical figure for Unite. Less than that means under 50% support from the members and more crucially only about one third of the workforce. If you follow this with an estimate of how many might follow through with industrial action, the figure drops even further. Not much point in calling for action if you can only mobilize about 15% of the workforce.
The trouble is that the scattergun approach of putting up 10 reasons for striking may backfire. Rather than bring in more votes in favour it could have the opposite effect. The absence of a single issue that all members can get behind is the biggest problem the union faces.
Apathy will play a large role in the outcome. I reckon that Unite will do well if they can get an 80% response to this ballot, with 63% of those voting supporting industrial action.