[Ace in the hole]

Your concerns have merit. Every electronic component will eventually fail, nothing lasts forever and after market parts can be more or less susceptible to failure. Depends on the company and where the part is made. Spal's controller looks like it has a pretty good heat sink on it already. I can think of a simple solution to a worry about being stranded though, a simple toggle switch to bypass the controller in the event of controller failure and feed strait 12v power to the fan and let it run wide open until you can replace or repair the problem.

The simplest thing to do in an electric fan conversion is to let it run all the time. You would still need a relay controlled from the ignition circ. to run the fan. I just like the idea of not having the fan run at 100% all the time when it's not needed and I'm not sure how loud it would be at full speed but I would think it would be loud enough to be noticable.