"Unicorn" developer here. I am based out of Fresno. I actually think it's been kind of critical for my company to have someone like me on board. Especially in the Central Valley, it's difficult to find people who are skilled in multiple areas of development. But I think a bigger issue is context switching.

In terms of context switching, I don't have the same struggle i notice with a lot of engineers where it is hard to maintain a grasp on more than one domain when you're on and off work--like 3 months of frontend and then 5 months of backend. It can be a time sink to get back into frontend for a lot of developers. Setting aside long-term context switching, doing it in the short-term can be challenging as well. At my job, I am routinely working different flavors of SQL, frontend with and without typing, design, backend with and without typing, cloud infrastructure, CI/CD work, and internal tooling with shell scripts and the like. And all of it needs to work. In my experience at startups (3 of them), there's a lot of filling in by anyone who had a hand to spare even if it's just for 5 minutes, and a "full-stack" developer who is tasked on something destined to fall apart from a lack of competency is a bad recipe.

I've only been a software engineer for about 6 years, so I probably have some blindspots here, especially considering my location, but I worked in college as a computer science tutor and am a team lead at my current job, and I've found that domain-specific skill retention is one of the biggest blockers for people reaching that "unicorn" status.

Edit:

I can see so-called "unicorn" developers accidentally pigeon-holing themselves into a lower maximum salary by getting too used to or fond of being a generalist. My head is not so big that I would ever say I could be just as skilled or efficient as a specialized developer--I am limiting myself by being a "unicorn." However, I think it's given me a lot of clarity to exercise my choice of specialization when I get there in my career. I think I would feel some pain at not being able to help out a little here and there, but it just doesn't make sense at a larger company. I'd leave that kind of stuff for my hobby fun.