I prefer Pathfinders to Rangers, the extra benefits they get are more then worth the extra points cost. Don't forget they become Scouts as Pathfinders.
As for why they are good against Multi wound units.
First, most of the multi wound models I've seen have good armor saves, good toughness or both. Pathfinders ignore toughness when they roll to wound. They also ignore they're own ballistic skill when they roll to hit, at least with the Long Rifle, the pistol they carry uses the BS of the unit. Pathfinders make they're weapons AP1 if you roll a 5+ on your to hit. You still have to make the wound. This means unless the target has an invulnerable save they are taking a wound. Instant death need not apply here.
Cover is essential for a Ranger/Pathfinder squad. If they aren't in cover they're dead or soon will be. Consider where you deploy them, and remember if you upgraded to Pathfinders they get a Scout move and ignore difficult terrain.
How many you field and how you divy them up is your perogative. Smaller squads can be easier to hide in terrain while larger squads get better salvos.
Close combat is their bane, you'll probably get to strike first as you'll be in terrain, but they're Weapon skill is garbage and you'll likely not do much against your opponent. In all my engagements in close combat I have yet to cause a wound in Close combat with my 5 man squad of Pathfinders.
Hordes of enemies will be a definite weakness if you don't have some troops to take them on. Rangers/Pathfinders don't shoot too fast so you'll soon be overwhelmed by the numbers. I tend to try and shoot bigger targets like Monstrous Creatures or harder to kill targets like heavily armored Terminators and the like.
If you've never used Rangers/Pathfinders I highly recommend trying them out. Maybe use a small 5 man squad at first for a while to get used to them. If they work out for you and you like them, go for more.
Good luck and happy hunting.
