Nubuck is a very low maintenance leather.
In general, smooth grain leather uses different products from hairy nappy leather (e.g. nubuck, suede, roughout).
Nubuck is smooth grain leather that is lightly sanded to a fine nap. Nubuck does not need to be polished, because scuffs and scratches don't really show up, since nubuck is basically pre-scuffed all over. Even if you wanted to, nubuck cannot be polished, because it's not a smooth enough surface.
While water stains are a common issue with nubuck, the nubuck that we use is waterproof, so you don't even have to worry about that.
Most of the time, your nubuck won't need anything more than a brushing, with a suede brush.
If your nubuck boots are absolutely trashed and brushing is not doing enough, we recommend taking them into your local shoe repair cobbler.
If you prefer DIY, the video below is a comprehensive guide to cleaning them, with product recommendations and demonstrations. The title of this video says "suede," but the information also applies to other nappy leathers, like nubuck and roughout.