Rising Voltage specifically doubles in power when the target is on terrain i.e. grounded. So running it over Thunderbolt does mean that for instance Specs Koko wouldn't be able to OHKO specially defensive Corviknight. (Presumably still worth it, but there is at least a small drawback.)
That's actually pretty notable for Levitate mons and makes it worse than Tbolt in several important cases (Rotom formes, Lati twins, etc.)