As of now, shields have no real purpose, but with the 1.15 update, functional shields have been announced, though the real question is how will this work. The current shield function in vanilla 1.15 would mess with the mod's combat system, the ability to just block every incoming attack with a piece of wood combined with some iron doesn't seem very lotr mod like to me. I don't know if there's something planned, but just in case, here are some mechanics I would change to balance it out.
Firstly (most likely the Mevans/ the team have thought about this), some weapons should have the chance to disable shields for a longer period of time. Weapons such as halberds, war scythes, poleaxes, and lances would tire out the opponents' arm due to their weight and strong impact, thereby disabling their ability to properly block for the time it would take to recover from a 75% speed weapon swing.
Some weapons like crossbows, battleaxes and warhammers could (on certain occasions) overcome the shield's ability to stop the attack and actually deal a percentage of the damage to the shield user. All the weapons mentioned above would deal substantially more damage to the shied's durability.
So far things are looking grim for the shield, but not to worry, there are advantages to having a shield, these are increasing the recovery time of the opponent's weapon after a successful block (with the exception of daggers and swords due to their lightweight and quick attack), the obvious blocking damage from incoming attacks, especially arrows, and perhaps even the ability to attack with one (shield-bash).
On the shield side, some few tweaks, adding in 2 new shields made of bronze and iron respectively, these would be upgrades to the wooden one, making for better protection against polearms, bolts and such as well as giving more durability to the shield. For anyone about to say anything on faction shields, it's stated on this page that the shields would serve as the texture for the shield, so no, no faction shields would be added (anyway they there would be no difference at all between a faction one and a regular iron-plated shield.
Please let me know what you think, would this be viable, or is it too far fetched? In the case it is, I'd also like to know your opinions on how two-handed weapons (large polearms (pikes, lances, longspears and poleaxes)) and bows would (or rather wouldn't) do when being used with a shield.