These can either be horizontal plays or product focused. For the latter it doesn't matter as much. For full stack developers domain knowledge is usually a lot more helpful there.

For horizontal plays this can matter a bit more. I run a well funded deep learning startup and we are starting to hire full stack developers next year.

I have thought about this a bit and we would be looking for people at a minimum who have dealt with some basic machine learning before. Much of the stuff in deep learning we do is displaying some sort of output from a neural network (eg: various ways of displaying a choice a neural net makes). Being able to do things like visualizing clusters is also important (this would be d3). The other part of this would be a basic understanding of being able to communicate with a data pipeline of some kind. We are java based but I'm imagining a lot of startups would be python based in this case.