Any time you get into your cases and start to move things around, there are risks. Anything from bent connector pins to broken assemblies to random bits of metal junk getting into the case can result in serious problems. It's best to decide on a configuration and then stick with it, from the standpoint of wear and tear on components. Even something as basic as a stripped-out rail hole or nut can prove to be a major annoyance over time. And as for random metal bits...it doesn't take much of a little shred of metal to cause a major disaster in any electronic device, particularly one where you have DC at a rather hefty amperage. I have not-fond memories of my father totally destroying a TV many years back when a tiny, broken-off bit of antenna wire not more than 1/4" long fell into the set's back and blew up something high-powered and important. Given that much of the circuitry inside a Eurorack system is exposed, including (quite often) the DC rails, this is a very real hazard. Also, keep in mind that everything behind that panel is interconnected through the bus boards; a failure at one point in a system can easily cascade into other modules, the power supply hardware, or even get out of the system via any number of connections to cause havoc elsewhere, depending on what failed and how.
So, yeah...economics again. Blowing up a build, though, is a disaster of proportions far worse than simply wasting money recasing something that's already cased.