you need 10x infantry or more then garrison to storm it. 10k in your besieging army for 1000 garrission so to speak. But for high amount of garrisson it can be bad idea, as it can decimate your infantry in morale and casualties.
Storming a capital fort with 500 garrisson is lot easier then storming level 8 fort with 4000 garrisson. But if you got the money to spare the mercs, why not.
AFAIK siege pips have influence on effectiveness of storming a fort, but i could be wrong.
There are a lot more assault mechanics than that. You can reduce the number of infantry needed by reinforcing, merging and consolidating.
Assault mechanics are a completely different beast, equal to land warfare mechanics.
It's just that nobody ever bothers looking into them because assaulting a fort is worthless unless it's already almost empty or you arrive after a month's tick and can take it with a barrage+assault combo in 2 days.
So to answer the original question of @Zuru84
It's not effective at all.
Unless you know what you're doing in which case it's a niche option that's rarely ever used.