I've been using two Digital Greenhouse ballasts with the small internal fans for years. I was weary too when I bought them but have had no fan-related issues, and one replacement incident in the past several years. To me they are not loud at all, and they are still more quite than magnetic ballasts. The fan is essentially just like a little CPU/computer fan in there and should that fail it shouldn't take a rocket scientist to fix it. It should shut itself down before it overheats.The DG's can be opened up very easily from the top and I blow mine out on occasion.
I've heard it before too that "should the fan die the ballast is junk", but I don't buy into this much anymore. For one thing, I've actually never had or heard of a CPU fan dying. It can\does happen, the lubricant can dry out or dust can accumulate and they can seize up, but this has never happened to me or anybody I know and I have a fair amount of experience with computers. I'd say small fans are less likely to fail than larger fans, in general.
This is just the way some digital ballasts were designed to handle heat, since the electronic components last longer the cooler they are kept. Conversely, the Lumatek design has the ballast components sealed in gel with an aluminum heat-sink body design. However, Lumateks do run a bit hotter in my experience unless you put them on another metal/heat conducting surface (they sell an external cooling fan also).
Lumatek ballasts might not have an internal for cooling, but the con to its design is that these ballasts cannot be opened or serviced readily. The larger/dual 600 watt Lumatek ballasts have an internal fan as well.
