As someone who's built a few engines and also worked with third party ones: It really isn't the hard part for a 2D game. High fidelity 3D, different story. But something like Stardew Valley, I'd dare to say custom engine and something like Unity is pretty similar in effort, considering that you need to deal with doing things in the engine's way, which requires workarounds and what not. Bringing it to many platforms gives the engine a head start, but I'd say it's comparable.

Iterating on the game content itself: _Insane_ amounts of effort, in my experience.