Federation forcefields are immediately effective against both directed energy weapons and projectiles. And we have yet to see an "individual" forcefield. Perhaps because the energy consumption is too big?
Worf is able to modify a com-badge into a force field in A Fist Full of Data's which lasts long enough to stop him being shot.
The Borg should come complete with a kevlar style bullet proofing included in their armour, and if they don't their nanites should be able to give them such a coating after the collective gets shot at a few times.
Trek changes how it's tech works from episode to episode making it impossible to work out why things are the way they are.