The general run of goblins are pretty much inferior to dwarf military with any sort of decent training and equipment.
- They are generally of middling training (I assume level 1-5 or so)
- They use inferior metal (copper, silver, maybe iron) for weapons
- They do not fully armor and thus leave limbs unarmored that can be easily crippled - even by wooden crossbow bolts
(This varies since the helm/cap and mail/breastplate mix is different by goblin civ)
- Their morale can be broken to cause them to flee
About their only assets are that lashers and archers in groups can be dangerous. Mass attacks with weapons that can penetrate/harm through armor will eventually not be blocked. And once crippled/passed out a dwarf can then quickly be bludgeoned to death.
Elite goblin leaders are a greater threat since they are much more skilled. The archers can shoot through fortifications and thus are just about the only unit that can hurt placed marksdwarves. The others have a greater ability to dodge traps, block bolts, and essentially give the military a greater challenge. And they can tear up lesser trained dwarves and recruits much as your elites go through goblin regulars. They are vulnerable however since they share the standard goblin weakness of not wearing full armor, and often "lead" units and thus hit things like cage traps first.
The equalizer (if anything) for goblins is mounts. They make charge attacks and can knock dwarves off balance. They act as additional targets and suck up bolt fire and clog traps. And, depending on the mount, they are arguably a much more dangerous attack threat than the goblin rider. Some of the auxiliary attack units (normally trolls) might do some damage, but are usually not that big of a threat. Though a unit of flyers, or giant cave spiders, might make one rethink attack tactics.
Sieges bring goblins out in numbers, but they also give you time to sound alerts, bring in your civilians, and do some preparation. And the goblin squads will hang around the edge for a bit of time before attacking - and even then not all at once. And besides a goblin squad running once it takes enough casualties (especially if the leader is wounded) the squads on the edge will leave if there are enough casualties taken by the entire attack. So, for instance, if you clobber the group of trolls and decimate the initial attacking squad the other squads might just decide to leave without even engaging. (Which leads to defense designs intended to trap as many goblins *in* as possible before they decide to start running.)
In my opinion the greater danger to your dwarf population are the non-siege ambush squads. Snatchers might cause a casualty or two, and stab some watch animals, but most civilian casualties are some poor woodcutter (or liaison) getting jumped by seven goblins out of nowhere who hack him up before the military can respond.
The fortress can take steps to prevent this, but there always seems to be a certain point in a fort's development a few years in where the appearance of 2-3 ambush squads will potentially do a lot of damage.