Let me deal with muzzle brakes on AR-15 type rifles first. Now, we all know that a muzzle brake directs part of the burning propellant gases upward near the muzzle. This is to offset the muzzle flip caused by the recoil of the firearm and allow for a more rapid recovery for follow-up shots.
Now, on an AR, the weapon is designed to significantly reduce muzzle rise by directing the recoil forces directly back into the shoulder. The bore line runs from the muzzle straight back through the centerline of the stock and into the shoulder. On many other rifles, the bore line runs above the centerline of the stock, and in some cases above the stock itself, accentuating muzzle flip. Besides that, the recoil forces generated by a .223 caliber round are not really significant in themselves. Added together, this simply means that there is really no need for a muzzle break on an AR-15 type rifle. In fact, they can sometimes cause problems, as I'll address in a moment.
Flash hiders are another story. Once again, we all know that a certain percentage of the powder charge of any round burns beyond the muzzle of the weapon. This produces muzzle flash. Most rifle ammo, including .223, is designed for optimal performance in barrels of 20 inches or longer [usually 22-24"]. Thus, when the barrel length is shortened, more propellant burns outside the chamber/barrel and causes a greater flash. While this is not usually a problem in daylight, in low light or darkness it can seriously impair night vision, rendering the rifleman virtually blind for a short period of time. The British noticed this problem when they introduced the Enfield No5 Mk1, a short barreled jungle carbine chambered in .303 British. It was essentially a cut-down and slightly redesigned Enfield No4 Mk1. The muzzle flash from the 20 1/2" barrel of the carbine was significantly enhanced over that from the 24" standard rifle. So, they incorporated a conical flash suppressor to direct the blast forward and not upward into the operators sightline.
Now, the AR-15, as with most short barreled rifles, produces a significant muzzle blast [more so in the 16" bbl versions]. It is not overwhelming, but it is significant, especially at night. For this reason, a flash suppressor is a desirable option. There are many designs of suppressors, three prong, four prong, cage, etc. All work rather well, but there is one thing to keep in mind when installing a suppressor. You want to install it so that the muzzle flash is not directed directly upward into your sight line. The less superheated gas and burning propellant crossing your sight line, the better for your low light vision.
I apologize for the length of this post, but I was bored. I hope that this helps you.