I think it's safe if the wheel is sturdy, and you don't push yourself beyond what you can get out of. For example, if you go for the 'flat-to-the-ground' position and have to twist and turn and pull to get out of it, you could pull a muscle.
It's a very basic device, but I think it is very effective, and you can challenge yourself more as your core gets stronger. (It is similar to ball roll-outs, and you can do a similar move with a barbell as well).
You can also hit the lower abs and hip flexors a bit more by rolling in and out with it on your feet (some have straps that you can put on your feet), doing pikes and knee-ins like Cathe does with the stability ball.