You can easily use SFML in 3D projects. SFML just won't help you with that but since SFML builds too upon OpenGL it's perfect for such stuff.
You could then use SFML for the 2D GUI interface and handle your own OpenGL stuff.
SFML is certaintly not for games only but since it covers all aspects a 2D game needs (Graphic, audio, input, network, system) it fits perfectly for that matter.