I wouldn't necessarily want to argue this assessment, as I feel a Nova Cannon fired at a planet SHOULD cause catastrophic damage, but I'm not so sure. Every once in a while, the Imperium reminds us that this is the grimdark future, and that they are really no better than anyone else on the "good people" scale. If things go south enough, the Lords of Terra, or the Inquisition, can pass the order of Exterminatus on an entire planet, consigning a billion souls to the other side. When they do this, they dip into a scret little closet they have, and pull out a terrifying weapon. This might be a cyclonic torpedo, able to ignite the planet's atmosphere, virus bombs, keyed to rapidly exterminate all life down to a cellular level, vortex warheads, which tear holes into the Warp, and suck things in, or even a magna-melta, which bores to the core, and makes the planet fracture. These are all rare, ill-understood, and possibly impossible to replicate methods of wrecking an entire planet. A fleet of vessels with special munitions might also be able to do this.
So, why have i just rattled off all of this stuff, which people already know? Because these are what it takes to blow up a world. Comparatively, Nova Cannons are commonplace. If all it took to bake the planet was a hit from a NC, from a single ship in space, the event wouldn't be nearly so big, and these arcane doomsday weapons would just be a relegated to a few additional conversation pieces in Prof. Farnsworth's laboratory.
A Nova Cannon would do collossal damage to a land-based target, and probably take out a city-sized span, underground stuff and all, no problem, but even it doesn't have the AoE oomph needed to obliterate the planet, the same as the Hulk doesn't.
Afterward, the issue might be "what would happen if you did do this?" Exterminatus is rare, and not something even the draconian Imperium often employs. If your RT were to NC a planet, or actually crack it, and mess it up, the Imperium would come and find you, and do the same to your ship.