Here are some points I came up with myself:
Digital painting:
- Flexible for lots of visual styles
- Lots of resources available for learning
- Scales decently
Vector art:
- Easy to edit
- Scales very well
- Relatively easy to animate
Pixel art:
- Classic "game look"
- Relatively easy to work with at low resolutions
- Very very laborious at higher resolutions
- Does not scale at all. Better have that sprite size locked up by the time you draw it.
3D in general:
- More work than 2D
- After the model is created and rigged, making animations is easy in comparison. No redrawing.
Poly modeling:
- Specifically suited for angular geometric looking objects
Sculpting:
- Easier to create complex models than with poly modeling
- Specifically well suited for smooth organic looking things