They probably do work somewhat.
The whole idea behind these bunion correctors (like these ones) is that you are supposed to wear them at night and doing so means that the toe is straightened. The evidence from published research is that they can reduce the angle by a few degrees after a month. No research has been done on a group of people for longer than a month so it is not known if any more can be achieved, though it probably can.
The issue with these that the forces from the shoe and from the way we walk that are producing the bunion or hallux valgus are substantial during the day and it is a bit difficult to think that wearing a splint or corrector at night when not walking around that this will magically overcome the angle, so the small correction after one month in that study is probably not surprising.
That small correct does not mean that they should not be used. As noted by this doctor, they are particularly useful at helping with pain in the joint and keeping the joint more mobile, so that is a good reason to use them. They are certainly recommended for that reason alone and the added bonus is that they may correct the angle of teh big toe.