Any union is only as good as its local branch/rep, really. Unison, as the second biggest, has lots of resources it can call on, including its own legal department as well as an agreement with law firms specialising in employment and equality law, but the majority of members don't get anywhere near that, because what they need is local representation, and that is where the local branch/rep comes in. There are officials, who are employed by the union, but they are somewhat stretched, and mostly only tend to represent and advise the big cases, which do need that legal advice, or they work with the local branch to set up a network of reps.
Can I ask why you left Unison?