The warp bubble collapses when you get too close to a large mass, so you wouldn't be hitting the planet at superluminal velocities.
However...
Whenever you activate an alcubierre warp drive the momentum and velocity vector your ship started with does not change, even as the warp drive moves the bubble of space containing the ship at a different apparent velocity.
If your ship is orbiting an ELW at, say, 8km/s when you go to warp, it will still technically be moving at 8km/s, plus the velocity of the planet around its star, the star around the galaxy, and the galaxy through the universe.
When you deactivate the drive you will still be travelling in the same direction at the same speed as when you fired it up.
Depending on how far you travel in the meantime, and therefore the difference in velocities between the celestial body you left and the one you arrive at, this can leave your ship travelling at dozens, even hundreds of kilometres per second relative to your destination.
On the flipside, if you choose your approach angle carefully you could use this starting velocity to put you into a perfect orbit around your destination without expending any fuel on a corrective engine burn after deactivating the warp drive.
The Frameshift drive in ED, when dropping out of warp, automatically compensates for this change in relative velocity to put you into a stable orbit around whatever you drop out nearest to.
This is why you can sometimes see the blue "warp vortex" moving sideways in front of your canopy when dropping down or transitioning from hyperspace to supercruise, as it's propelling you in the proper direction with the proper velocity to cancel out the momentum you had from when you activated the drive to begin with, and making sure you're moving along with the body you now find yourself in front of.
All that explanation is to get to the point of your question: Imagine what would happen if the drive failed and did NOT correct your relative velocity for you when the warp bubble collapses.
Your ship would find itself in normal space now with the velocity it had when leaving your starting point which, as mentioned above, can be dozens or hundreds of kilometres per second compared to the body you arrive at. Depending on orientation you could be flying
away from your victim planet at that speed, but for the purposes of the question we'll assume you're flying straight towards it.
Your ship would essentially become an incredibly fast moving asteroid impact event. Depending on the material strength of the hull and the kinetic energy involved, the ship would either detonate violently upon reaching the lower atmosphere with the force of a small nuclear bomb, similar to the rock that exploded over Russia a few years back, or it would survive long enough to impact the ground with similar force and create a crater hundreds of meters wide. This is assuming an impact velocity around 20km/s or so. At speeds of over 100km/s or so you can simply remove the "small" prefix from nuclear bomb.
If you were to warp across the entire galaxy and then have a drive failure in front of a small moon, the huge difference in velocity and the energies involved would likely be sufficient to completely pulverise the moon, vaporising much of it and scattering the rest across the system, as if it were hit by an enormously powerful thermonuclear or antimatter warhead.
Hope that helps, and remember kids, always check your FSD integrity. Only you can prevent extinction level events
