Low poly modelling is really easy to do/model, but if you plan on having hand painted assets it becomes more difficult unless you know kinda how to draw ex: 
).
But if you only use pallets for flat colors and make good use of lighting/post processing from unity you can make it look beautiful too ex: ..
.
You can take a look here, it's an interesting source of information imo https://sundaysundae.co/how-to-make-low-poly-look-good/ .
As for the 3d unity programming.. it can be a little overwhelming for beginners if you plan on having complex systems, you can still try it if you think you have enough time, after all you get more experience from it even if you don't finish the game completely.
I heard that Godot has some issues with 3D lightning and definitely lacks good docs. That's why I am thinking about switching to Unity although I have little experience in it. Thanks, I will watch some low poly tutorials then will have to decide if Godot is reliable enough.
