I would be very curious to hear about your favorable experience with untested failover systems. My last job we tested our active/standby failover like clockwork every 3 months. It served us very well.

It's sort-of a saying in the industry that can be applied to many things. The biggest problem is the people who might know how to make it work are no longer there or have forgotten. If it's automated there's a chance it might work, but if it's slightly off then it's even harder to comprehend and adjust.

I've seen backups and failovers not work so many times that it's an amazing surprise when one actually works--usually after laborious manual off-script intervention and invention. I was being somewhat kind to failovers, I have seen smallish backups work on several occasions. An untested disaster recovery is the worst.

Edit: think of an unexercised procedure as not "It works for me" but rather "it worked for me once."

An untested failover system is indistinguishable from a box burning power and doing nothing.

On a failure, it might work. But there might not be a failure. Same difference.