I plugged in the attic fan the other day since we had a week of highs in the 60s. It did its job, but I noticed it wasn't blowing as hard. So, I grabbed the unit, brought it down and sure enough, one fan was barely spinning and the other wasn't at all. I lubed up the motors with some silicon spray and they went as expected after being powered up.
My one complaint about this thing is it could be quieter. The fans I've always had are used and worn out. There is some unbalance going on, and they're really just cheap fans. So, there is vibration transmitted to the chimney and thus the entire house framing. When I went to reinstall the unit, I grabbed some adhesive backed foam, and put some around the outside edge. Its not ideal as it is open cell foam, but it did some good in quieting things down.
If the the fans keep dying on me, I'm thinking I'd like to swap out the 3 fans for probably a 6" or 8" inline centrifugal fan like the one below. I can hang it so it doesn't vibrate anything, and use flexible ducting to go to the chimney.