As Austin says, unless setting the health or armor at spawn time, the problem with forcing a player's health or armor server-side without notifying the client of the changes is that the client's data will be wrong compared to the server's. This can lead to bizarre behaviour, such as the client believing the player is dead, and starting playing the death animation, then receiving an update from the server telling it that this player is still moving and shooting stuff. You will then see a wandering corpse farting bullets.
When you set the health or armor of a client arbitrarily, don't forget to send him a "Health" or "Armor" network message. The shape of such messages is generally standard between all MODs.