The answer to the thread title is very simple:
Yes, it is very limited. AFAIK, SmartDashboard 1.0 hasn't really received any new features since its introduction, and it's annoying to extend due to limited documentation.
You might check out SFX (SmartDashboard 2.0). It's a lot shinier, and has a lot more features (including sliders and such). However, still limited documentation for extending it.
Both of them can be found in your ~/wpilib/tools directory (SmartDashboard.jar or sfx.jar) on a computer with the eclipse tools installed.
Self-promotion: Of course I'm biased, but we write our dashboards
using HTML+Javascript. It's a lot more flexible, and easier to extend as you're only limited by your ability to create shiny javascript. Only downside is that you can't connect to the built-in wpilib camera stuff, but that code is fairly terrible anyways. We use mjpg-streamer instead (search the forums).