Yes, they can. In fact, I think it's Zilla that makes a "rheostat" which is essentially, a dimmer. It works on the principle that it regulates the electricity into the device, therefore only powering it up a portion. The more power your device uses to operate, the more range of control you have. The tiny size undertank heaters use next to nothing, so their range is not nearly as variable as the largest one.
The advantage is that they offer is a constant temperature, rather than on/off. The disadvantages are many, such as, as other posters said, you have to have a thermometer to monitor the temperature. If you use one, remember that your tank can take 6 or more hours to come up to full temperature, so, to me, that's something you do on a day off when you can monitor it for hours on end.
Also, as other posters said, the temperature of the house does not affect them. If you, say, light a fire in the woodstove, and the house gets up to 80, then you are adding heat to 80 degrees. If your heat stops working (power failure, equipment malfunction etc.) You are only adding the same amount of heat as you were when the house was 68-72.
Keep your thermometer on at all times, and enjoy!