Unions in general are A Good Thing. We should remember, though, that unions' job is to advocate for their members. The idea is that unions push for their members, parents advocate for their own and their children's interests, and the government pushes for its own agenda, and the push-me-pull-me between the three elements is supposed to create the agenda for education. Kind of like governments having more than one debating chamber.
The union view is about the interests of members, not The Truly Enlightened Truth Of How Schools Should Be Run. Provided we all bear this mind, unions' voice is very useful and important.
Unions only start to be an issue when the "other" sidethe one pushing against unionsbecomes too weak and is not able to get its own side across.
Example: In America, the highly decentralized nature of the education system creates dysfunction-teachers' unions are bargaining with fairly minor local official rather than central government people, which gives unions arguably way too much power, leading to a lot of really insane demands. At the start, many American teachers were demanding the vaccine before they would agree to teach in-person again, which given the early approval of the vaccines is actually pretty doablebut as soon as it became clear that teachers could indeed be vaccinated at an early stage, the unions in many places have shifted the goalposts yet again and are now demanding, in many places, that kids also need to be vaccinated before in-person school can start. Widespread vaccination of children won't even begin until 2022, so this is absolutely insane.
I don't think the UK situation is much like this at all. Unions here have to argue with our fairly powerful central government, meaning that you tend to up with policy being dictated by a balance of different interests. But I do think the US situation is a good example why you don't just assume "Unions are lovely because we should Support! Teachers!" and automatically follow every demand. If we did that, we'd end up with the US's dysfunction.