Glass tube heaters are just glorified light bulbs. You run electricity through an element, and it makes heat. Just like a light bulb. In this case, of course, the element is selected for optimal heat output rather than brightness. Now, your light bulbs get changed every year, possibly even sooner. But, do they all suck too? I think we are just a tad hard on heater MFRs (and no, I have no affiliation with any).
It has been repeated often above, and I love the idea. Just buy two undersized heaters, and the chance of boiling your tank are greatly reduced. Replace the heaters when you replace your "other" lightbulbs, and you will likely never have problems.