For our reverb/echo/chorus effects, the only advantage is an additional 2.5 dB of dry-signal headroom. This is imperceptible under most circumstances; however, it may be appropriate for a line-level effects loop.
For our other effects, there is no advantage to using 12V.
Under no circumstances should you power our effects with more than 12V. While there is some protection against higher voltages, long-term exposure may cause damage.