Both are hard but the shape of that hardness differs.

The frontend problemspace didn't feel like it was depth, more breadth... it's a constantly moving surface and lots of unreasonable demands were created (pixel perfect layouts being the most telling of them). However the number of things you really need to know are few and it's the tooling that changes often.

The backend problemspace is typically not as wide, but far deeper... the depth looks like still dark waters, they run very deep, you think it's smooth sailing but it's not. The tooling changes seldom and things are a lot more deterministic and within your control... but now you've exposed yourself to a lot of things you didn't previously have to be concerned about, the kernel, networking, memory allocations, distributed systems, algorithms, etc. The tools may remain the same for time spans of decades, but the depth means in your entire career you will not exhaust going deeper still.

Both are hard, but the shape of that differs.