You are obviously not an engineer! Due to this, you have no idea on how to calculate failure rates.
If e-fans were as reliable as mechanical fan/clutch assemblies, you would not find articles like these:
http://autorepair.about.com/cs/troubleshooting/l/aa032903g.htm
http://www.caradvice.com.au/1733/volvo-xc90-and-xc70-electric-cooling-fan-recall/
http://www.popularmechanics.com/how_to_central/automotive/1272431.html?page=2
http://edgemotorworks.com/index.php?option=com_content&task=view&id=25&Itemid=55
...more information, see if you can comprehend:
Cooling Capacity:
By far, the most common misconception is that the electric fan cools better then the stock. People see the high CFM numbers in the catalogues (for example, the Black Magic is advertised as pulling 2,800 CFM, and the Perma Cool "finger chopper" is advertised at 2,950 CFM) and are impressed. The truth is that no one actually knows the CFM rating of the stock fan. It has simply never been measured by anyone in the aftermarket and listed. All we do know is that it is rated at "a lot", more times than not it will exceed 4000 CFM. And to be honest, who cares how much air the stock fan moves? It is more then adaquate as long as it's clutch and the rest of the cooling system is in good shape. The steeply raked blades of the stock fan move quite a lot of air, even if it turns slower then an aftermarket electric unit. Experience has shown that when in good shape, the stock fan is even easily capable of handling a 400HP engine. How much power do you make?
Efficiency:
Of course, you're not necessarily after better cooling. You've seen those TV commercials or catalogue ads that promise "Up to 17 free HP!" by converting to an electric fan. Well, that's simply not true. There will be a horsepower gain for sure, on the order of 0.5 - 5HP, with 17 H.P. reserved for big V-8s using 40 lb fans at 6000 rpms. But remember that the electric power to run the fan must come from somewhere, and that somewhere is the alternator. Electric fans draw quite a lot of current. Most pull surges of 35A or more to start up, then run at 8A-10A. This puts extra load on the alternator. Ever pedaled a bicycle with a generator powered light? If so, then you will know that as the electrical load on a generator/alternator increases, the generator/alternator becomes harder to turn. Suddenly, all that "free" HP you just freed up is once again being used. This time, though, it is being used to turn the alternator. There is also an inefficiency in the whole system. We are converting mechanical energy, to electrical energy, to mechanical again to run the fan. This very inefficient, much less then just driving the fan directly via the mechanical energy of the rotating shaft
Reliability:
Lastly, there is the question of reliability. The stock clutch fan can have two failure modes: the clutch will fail, or the fan will physically break. The electric fan introduces many more failure points: fuse, all wiring connections, physical failure of the fan, failure of the motor, failure of the thermostat
And to answer your last question, I know better than to buy an e-fan!!!!!