I used to making milk from sunflower seeds and I always add the same amount of xanthan gum. I made my first raw quinoa milk and the water separated - but just a bit, maybe 1/4 of an inch - despite the fact that dry amounts of both sunflower seeds and quinoa and resulting liquids were pretty much the same. Next time I added a bit more of xanthan gum and it was ok. I'm thinking that could be because sunflower seeds have higher fat contents? 36% vs 2%.
Some plant products carry a good amount of lecithin, which is itself a potent emulsifier, so there is less need to add an extra emulsifier. Soy beans and sunflowers are both commercially exploited sources of lecithin isolates, so one can assume they have a lot.
Lecithin is also contained in eggs in a high concentration, and the reason eggs are often helpful in mayonnaise making - BTW, eggless mayonnaises can be homemade from soymilk, though not as easily as traditional mayonnaise.