The hard truth: real milk froths best, the fresher the better. Nespresso also notes that protein content should be high as this leads to better frothing. Low-fat milk creates dry, stiff foam while high-fat or whole milk creates wet, silky foam. Milk that has previously been frozen will not froth.
Many reviewers across the models have noted that oat milk has worked and led to frothy results, but milk alternatives like almond, coconut, or soy milk often have a harder time yielding consistent results. A lot of it comes down to experimentation, unfortunately. We will note, that over the years more companies have released alternative "milks" that are getting closer to the real deal in terms of texture and what we'd call frothability.