I don't have any experience of using nematodes, but I do use this.
It is approved by the Soil Association, is organic, and is based on ferric phosphate, is water resistant, and breaks down into fertiliser. And it can be used 365 days a year, whereas nematodes only work April to October, and has no special storage requirements:
What I don't understand is why it is not better promoted as a cheap convenient alternative to metaldehyde slug pellets. Is there something I am missing? Is there a problem with it that I am unaware of? Or has it just got a bad name by association - blue pellets etc.?