The part where Spock's burial tube "must have soft-landed" due to "gravitational fields [..] in flux" surprises everybody. Is it because
a) Spock's coffin was expected to burn up in atmospheric entry?
b) Spock's coffin was not originally aimed at the planet at all?
c) Spock's coffin would in normal circumstances have been buried deep in bedrock, even if still intact?
The first option would indicate that the coffin, despite being an explicit Mk IV photon torpedo casing, would be of feeble construct. Bad news for Kirk and friends in ST5, then, as terminal guidance by physical means wouldn't have been possible with the torpedo that hit God and gave the heroes time to escape. But in that scene, and in many others, torpedoes and comparable devices indeed accurately hit planetside targets, typically exhibiting the classic glow while doing so:
So, guided balls of energy after all, leaving the feeble cartridge in the launcher? Not necessarily even in option a). After all, the very glow (from the drive or whatnot) may affect survival in atmospheric entry, and the glow from Spock's coffin wasn't of the classic torpedo type and in any case appeared to dim down before the torp hit the horizon (or the atmosphere) and the lightshow was taken over by the sunrise.
That the casing would be so easily combustible (whether in flight or at launcher) is not my favorite interpretation anyway. But the durability by which they enter bedrock or the outer layers of a star may
be due to shielding alone. FWIW, shielding is quoted as the explicit means of survival in "Half a Life".
In any case, a photon torpedo is a valid means of physically delivering a payload to a destination in dozens of episodes using the terminology and doing the VFX. That's a trick energy bolts would have great difficulty pulling off. (Greater, perhaps, than phasers, which for their part use terminology curiously similar to that of transporters, and may indeed be weaponized transporters of some sort.)