30 years ago, every app developer was "full stack", and the concept didn't even exist. As apps moved to the web during the dot-com era, the added complexity made some companies split their teams of entry level developers into front-end, back-end and database. Still, at that time, this "stack" was simple enough at the time that a good dev could learn the basics front-end, back-and and basic db work within a few years. So, a few years after dotcom, the "full stack dev" became popular, but really what they were delivering was the same as the app devs of the early 90s.

But even within software, engineering is so much more than just making web apps or RESTful microservices behind the web apps, and the field is still grown rapidly. There is data engineering, machine learning engineering, infrastructure ("devops") engineering, operating system engineering, driver engineering, network engineering, real time system engineering, algorithm/library engineering, and I probably left most of the sub-fields out.

So my main issue with "full stack engineers" is that I've met a few with (in reality) a fairly limited skillset who are quite overconfident about other aspects of software and the infra around software, who assume that the principles they follow when buildings web apps apply to all domains of software, and show a significant amount of hubris [edit] but still often failing spectacularly when trying to use their particular skillset in a somewhat different domain.

I realize that this impresson of mine is based on a few bad apples. But I think there is something in this "full stack" concept that can contribute to some devs underestimating the the kinds of work that are normally not considered part of this stack.