It could be micro solder crack, or a connector on the reverb card. However, I have heard of the reverb going on these heads, as well as MIDI switching. The more complex, the more chances for failure. That is one reason why I like the original Legacy, though it lacks the third channel drive, and master volume, of the 3.
I'm not well-versed on the internal card; never had my amp apart. The V3m uses the same card, I believe, and possibly other, more recent Carvin amps, as they pretty much switched to the digital version. There was talk that the module itself was also the same as found in their FX processor, but tamed to only provide reverb. If I had the equipment to pick apart and analyze, I could give a proper answer. As for the Legacy series in general, the FX loop driver is prone to overheating and failing over time. The rest of the amp is pretty solidly built.
Worst case scenario; you can't use the integral reverb, and need a pedal/unit to provide that.
Apocalyptic scenario; the amp fails, and you want another. Get a Legacy 1 or 2. The 2 is the last popular, as it supposedly sounds different. I haven't heard one. Or, get the preamp pedal, run a reverb pedal after it, and then a power amp.
Basically, the reverb/MIDI potential failure is the only caveat I have about the L3. The rest of the amp is built just like the original, which has obviously survived.