Long story short, the one extra dimension isn't just "a little" harder -- I'd estimate something close to an order of magnitude more difficult, once you account for the inherent, issolated increase in difficulty, the additional difficulty of visualizing problems/bugs in 3D vs 2D, and the combinatorial difficulty of making all those 3D systems work together.

Then there's the art assets -- which have to be visually correct and appealing from literally any angle; not to mention that 3D typically requires more complex shaders, textures, and combinations thereof.

2D is significantly simpler.

Long story short, the one extra dimension isn't just "a little" harder -- I'd estimate something close to an order of magnitude more difficult, once you account for the inherent, issolated increase in difficulty, the additional difficulty of visualizing problems/bugs in 3D vs 2D, and the combinatorial difficulty of making all those 3D systems work together.

Then there's the art assets -- which have to be visually correct and appealing from literally any angle; not to mention that 3D typically requires more complex shaders, textures, and combinations thereof.

2D is significantly simpler.

This. 3D is as far from 2D as 2D is from a text adventure game.

Back in the 80's (and even for most of the 90s!) one person with a bit of artistic talent could churn out a successful 2D game (a shoot-em up for example or a good platformer) in a few weeks to a month in their bedroom and become comfortably rich. Just remember Jeff Minter, for example.

Those times are beyond most peoples reach if you aspire to 3D and want to do it all yourself. One of my work collegues is also a game developer in his spare time and two of them are trying to create a huge scoped FPS. Just to put things into perspective, if it's something youre not used to, it will soak up a lot of your time. He is a programmer by trade not an artist, and it took him several hours to just texture one small model.

If you really want to go 3D, you should consider buying or using free assets, keeping the design small and simple, and planning before you start. As smart people often say: "Failing to plan is panning to fail".

If you go 2D, and you have anything more than the ability to create 'programmer art' (which most people do, given time) you can produce more, faster, and get stuff done. Your game might not be the next call of duty or the next skyrim, but it doesn't need to be. It's unique, and its yours.

Good luck!

Frankly it all depends.

3D math and asset pipeline is certainly more complicated than 2D.

However using an engine that does all of that for you makes the difference much smaller. In fact, 2D can actually be harder in some engines. Starting from scratch though the difference between a 2D game and a 3D game is massive.