front-end: webpack/parcel/etc. configs, plugins, css-, js- and asset-'transpilation'. arcane CSS knowledge, keeping up with HTML developments, learning about promises, no async, no web-workers, react hooks, etc.
back-end: perhaps less, uh, dynamic, but: db architecture, SQL, much more CLI/linuxy stuff, server security, API design, version control, deployment, containers, ansible, load-balancing, nginx config...
The difference not primarily being a sort of 'core' skillset, but rather purely learning and remembering the intricacies of all these things. That's too much for most people.
EDIT: I find that there's a definite sense of 'progress' or 'leveling-up' in all these areas put together. I get better at finding the right documentation, finding the best places to go for help, figuring out what the foot-guns are as well as distinguishing fads from truly worthwhile developments.
I get better at 'intuiting' where I might need to take a deep dive to truly understand things, or where I might need to consult an expert.
It gets easier to take lessons from one area and apply them to another.
But ultimately there's still so much 'domain knowledge', much of it shifting and changing, even if just because the client cares about it, that it's hard to keep up in both the front- and back-end world. And even if that shouldn't have to be necessary, in practice it often is.
For example, I've been doing more back-end stuff for the past few years. I keep an eye on what's happing in the js/React world as that is what I mostly did before. I struggle with setting up a new project because apparently Webpack changed, apparently we do CSS in js now, and apparently hooks are the hot new thing. I have a /lot/ of time to study all of this, but unless I actually work on a project that uses all this fancy new stuff, it doesn't really properly become part of my skillset.
The same goes for backend stuff. I can set up proper Linux servers 'in anger', but apparently now it's pretty useful to know how to use Amazon/Moft serverless stuff and I just never got to that. And I'm single and almost permanently reading up on all this fancy new stuff!