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Full-stack developer conjures up a tech superbro. One person juggling front end, back end, data storage in three flavours, DevOps, CI/CD and a bit of machine learning on the side. What the job description is actually describing is a jack of all trades, and we all know how that one finishes.
Nobody masters every layer. There is too much of it, and each layer has got deeper every year I've been doing this. So the full-stack developer ends up a generalist who can move through several domains and go deep in none of them. AI makes it worse, because it hands you a plausible answer in a domain you don't know well enough to check.
The appeal is real. They prototype fast, they bridge teams, they can pick up whichever end of the project is on fire this week. The cost shows up later. They build a front end that works but is miserable to use on a bad connection. They set up a database and miss the indexing and partitioning that a specialist would have done on the first pass. They ship an API with no caching, no rate limiting and no input sanitisation, because those three things only look obvious once you've been burned by them.
The well-rounded specialist is the better model. Deep in one or two areas, working knowledge of what sits either side, and enough self-awareness to know when a job has gone past their depth. That last part is the whole thing. A front-end developer who knows to pull in a back-end engineer before designing the API contract, a back-end developer who defers to someone else on authentication and encryption, and the work lands better and often faster than the one person doing all of it alone.
Without that depth you get output that is functional and unremarkable. The specialist spots the trap in the design review. The generalist finds it in production, six months later, with a customer on the phone.
I'm not saying the unicorn doesn't exist. I'm saying the few people I've met who came close got there over a career, and none of them got there because a job advert asked for it on day one.
The unicorn isn't an aspiration. It's a job title that asks one person to be mediocre at six things instead of excellent at one.