I read somewhere someone saying Torpedos cannot be used for Orbital Bombardment, I just would like to understand why?

There are multiple places in the lore mentioning torpedos being used, like in the Nids Codex for example (see the Carnifex that survived a exterminatus from Cyclonic TORPEDOS, just long enough to be killed by a Plasma Torpedo hit from orbit).

Also, how ballistics tests and the like should be changed for Torpedo OB?

Personally, I consider torpedoes an effective way to bombard shielded cities from orbit, and a reasonable way to add solid firepower to a more conventional bombardment scheme.

For the tests, I would treat it exactly like shooting at a ship (defensive turrets, getting through shields, ect), but modify the roll based on the scale table, with a void ship sized target being the +0 in the middle. A hive would get +20 or so to hit, while trying to plant a torpedo directly on the head of a grox would be in the range of -60 or so. I'd also factor in degrees of success/failure. Even with a degree of failure, the torpedo probably still landed in the correct field, and the grox would still be very dead.

I have chosen to understand things this way:

Ordinary torpedoes cannot be used for orbital bombardments, because they would burn up in the atmosphere upon entry.

They move very fast and while somewhat armoured, are designed for the air-less enviroment of the void.

Certainly torpedoes can be desgined that can survive re-entry, but these are much more expensive, as they need to be made from (or atleast coated in) certain ceramic materials to protect them from the heat. While such torpedoes could be used in void battles, it would be rather wasteful, as the torpedo is rather more expensive.

This however is a reationalization. An attempt to create an explenation. Not something necessarily sensible.