If you write a small app that is only used by a few dozens to maybe hundreds of users, then the backend can be easier than frontend. You can simply code one backend and put it on a fast VM and be done with it.
If you need to scale to thousands or millions of users, things can get messy, because you need to scale horizontally, which leads you into distributed systems territory, which is quite complicated stuff.
If you write a small app that is only used by a few dozens to maybe hundreds of users, then the backend can be easier than frontend. You can simply code one backend and put it on a fast VM and be done with it.
If you need to scale to thousands or millions of users, things can get messy, because you need to scale horizontally, which leads you into distributed systems territory, which is quite complicated stuff.