I too have struggled with the idea of the full stack developer for years and years so found a fair bit of sanity in this article by Chris Ferdinandi.
Most of the article is highlighting what Charlie Owen tweeted in this thread. I almost found myself cheering allowed to each tweet.
Here's a taster:
Do I consider myself full stack? No! My generalist knowledge exists to assist my core skills.
There’s a reason why we don’t have handymen building houses. They might know a bit of everything, but you don’t let them build a structure meant to last centuries.
Similarly we don’t let generalists build robust sites and apps that are meant for all.
Read the entire thread if you can. It's brilliant stuff.