Is there any reason why the arduino uno couldn't be run 24 hours a day 7 days a week for several years in exterior environment? (mounted in an ip66 enclosure). Most people use PLC's because they are robust and can be run this way. Are there many arduino users that have used it to run all the time without issues? Thanks.
I will add to the comments else-thread by reminding you that the Uno is, primarily, a prototyping development environment. So, while reasonably robust, the primary goals of the platform are not necessarily guaranteeing good (or even consistent) MTBFs under rougher field conditions.
This is not a dig at the platform, but merely recognition that an individual board may not perform the way you expect when run 24/7. This is especially true if it is under a constant load of some sort. You also need to consider your own circuitry (are parts being run close to their recommended ratings, are you relying on parts that are known to simply age poorly, etc.) and the conditions under which the unit will be operating (i.e., how damp, how hot or cold, how much shock and vibration, how fast it can swing from hot to cold, etc.)
That being said, it is usually passive components that fail. Standard electrolytic caps, for example, dry out pretty quick.