It's relative, each game and each developer is different.

Examples:

-If you are a programmer, making art for your game may be very complex and that part may cost you more.

-Maybe you are an artist, but you don't know anything about programming and no matter how easy some tools make it. At some point you want to do something more complex and you don't know how to do it and you get stuck.

-You are a good artist and you know how to program, but you don't have the slightest idea of design and creating the level design is very complicated for you.

-You are good at developing games, but you do not have a community or friends to help you and therefore the beta-testing and feedback stages are very complex for you.

etc, etc, etc.

There is no part of development that is equally difficult for everyone, since each developer or team is different and has its own weaknesses and strengths.

What I can tell you, with current tools, making a game, you can do it in an afternoon if you want. But making the game that you have in your mind and making it a good game, that people like, is something complex that requires a lot of work, the tools help you, but they don't do the work for you.

Well, it depends on engine vs video game size+quality you're aiming. If the latter is high but you don't have required engine and assets to create your desired video game, it would be hard to develop that game. OTOH if the latter is low and you have proper engine and assets, it might be easy. Easy doesn't mean it can be done quick cause you'd still need to actually do the developing part.

I could add skill to the formula above but I believe you get the idea.