- scale - a frontend shows one user's view at once, a backend may need to be updating state for millions of users without slowing anyone's experience down. But these sorts of things are somewhat different than just "backend code", it's often closely tied to infra and data designs, which doesn't necessarily look like "backend coding" but is critical to make the rest work.
- business logic complexity - often the backend teams are tasked with working with frontend to figure out a simple, user-friendly API surface for the app and also to figure out how to make that work with the various not-written-down, not-even-fully-enumerated business logic permutations and backend-data permutations that could be going on. If they do their job right, the frontend can show different screens with values from the API ready to plug in for a particular scenario without having to worry about the combinatorial rules behind the scenes that led us to want to show the user THIS screen instead of any other one. The difficulty there isn't a mathematical or "hard" one, it's that the product team probably has no idea just how gnarly the set of business rules they sketched out actually gets. A backend has to support everything the frontend client can do today, but often ALSO support a bunch of things the public frontend apps don't yet do (whether this is for admin tasks, or for future readiness, or what).
I'm not sure I see the significance of the number of symbols in UIKit - you usually don't need all of them, I assume, and I know that there are some common patterns that get re-used a lot. Is it so different from in backend when there are dozens of databases you could choose, and a bunch of frameworks for each of a bunch of different languages you could choose, but for most problems it's not going to make a huge difference?
One thing that often hides the difficulty on backend is that the frontend team might be interacting with just one or two backend teams that then interact with a different set of teams behind them, etc. But the true "backend vs frontend" question has to include ALL of those teams, even the ones you don't directly interact with, since they're all in the critical path.
"Hard frontend problems are harder than easy backend problems" - ok, sure. But that's not a very interesting comparison.