Birthright:

I believe it reduces morale by one level.

Also it reduces province level by 1 and holdings (law, guild, temple) levels by 1 if they were maxed against the province level (if not maxed, they don't drop).

Pillaging is the less-targeted, but more direct way of removing enemy income and opportunities. It is less efficient in action economy, but provides an immediate gain to the pillager and an immediate and lasting loss to the pillagee.

Say it is a level 5 province. It would take 5 actions to pillage it down to 0 (and all attendant holdings). These would have to be rebuilt via Settle/Rule actions.
Occupying the province would take 1 action. Reducing its holdings to 0 would occur in the second action. Province is intact and able to be contested at some point.

Occupation ties up troops, and conquest leaves a vulnerability. The main advantage to pillaging, and the reason why the Unseelie use it so much, is that it allows the army to leave quickly without exposing its leaders to regency loss, while the target will remain impacted for decades. It's very difficult to get province levels back during this 'ice age' scenario, since only 1/2 of summers are eligible times for it, and 3-5 seasons aren't summer.

My point is that the game rules do not define it. As one of the newer players items like this not being defined has been rather frustrating. If Juan is updating the rules post, it is a good opportunity to flesh out the rules for pillaging.

A majority of the rules were originally from the rulebook. Over time, as the game changed, the rules which were changed/modified were listed and slowly codified.

That said, yep, Pillaging and Province Levy could both be updated

What about occupying your own province? It does not really make sense to have that take as long a time as moving into a hostile province and start taking control.

While I would love that, it does change the dynamic for war. Occupying your province can be as difficult as taking hostile ground, particularly when the law and temples say are against you

It's a little easier, since you can move your units to the province before using Declare War. Normally that would be impossible (unless you're betraying an ally). So it only ever takes 3 war moves to occupy your own province, while it normally takes a minimum of 4 to occupy an enemy province.

I think the reason it's difficult is because you're replacing your civilian law enforcement with your army. The only thing that makes that less messy is the lack of resistance, it's still unpopular and disruptive.

It takes three to occupy an enemy province (again presuming the effort is not disrupted). The movement into the province initiates both the battle and attempt at occupation.

That's not right. It shouldn't be possible to begin occupation until after the unit is in the province and the enemy is defeated.