If you have a larger heater than is rated for your tank, a malfunction could be much more devastating. A 300w heater that is "stuck on" will heat your tank up too hot way faster than a smaller heater of course.
However, if you have a heater in a brand you trust, I don't think is is a problem at all, but I have had el cheapo heaters stick on and cook a tank before.
I have the Won Pro Heat, and I really really like them. I have heard that the ones with the digital control can be a little iffy, but I have had mine with the "regular" knob for a while and they all are reliable.
I have been shocked by cracked glass heaters 2x, and I only have titanium now.
I have the Won ProHeat and so far have found the digital to be extremely accurate. The display completely jives with my thermometer and I have zero fluctuation. It does not wait or the temp to even fall 1 degree before cutting on to maintain the temp. I am impressed. Time will tell how this will do in the long run. Good luck!