I may be just lucky, but I have dozens of Tetra heaters (non-adjustable) that I've never had issues with. My choice brand is EheI'm though (though I've heard some people are having issues with them, not sure if they've changed them recently).
I love the EheI'm heaters I have one in my 5.5 and it does an amazing job keeping the water the right temp.
My fav is EheI'm for my 20 gallon, it costs a lot but works great for the next years to go. Top fins new heaters aren't bad if you are cheap (*cough* me)