I've used one in the past with no problems
Well, well! Perhaps I spoke too soon! My first brew with the Fluval 100w heater was fine, but I've just got a stuck fermentation with the second one. Mind, you circumstances weren't ideal so I'm not at all sure that the heater is implicated.
The day after fermentation started, we went to Edinburgh to pick up a new dog (rescue GSD :)). On the way back, we hit a stag full on, which wrecked the car :-(
Fortunately, we're OK, but it rather took my eye off the beer! So, I never roused the beer, which I usually do (Brewlab E Mid 1 yeast). I never checked the temperature (heater was set at 20C). It was the first brew with a fresh yeast slope, and I'd been having trouble keeping the starter at temperature (I'm in an old wooden house with no central heating!). So things weren't ideal at all!
I eventually looked at it yesterday, with a view to rack it into 2nd FV (it had been in primary for 6 days) and found it was pretty still, and at 1027 (OG had been 1044). There was still a respectable krausen covering all the beer, and the trub/yeast deposit at the bottom was emitting gas bubbles, but only very sporadically. Today it seems pretty dead.
Stuck fermentations are a real rarity for me, so I thought I'd post this now. I'll try another brew with the same yeast and heater soon & update on how it went.