As for the Monk, it's not designed for straight-up confrontations. It has better hit points, saves, and mobility than a Rogue, so it can potentially last longer if forced into a fight; but that's not his role. It is a good infiltrator and scout; it has a decent number of skill points; it can operate incognito (e.g. passing as a peasant) effectively even without weapons nor armor, and therefore act as a spy. This role isn't new; it was designed this way since the AD&D 1e days.
I have played a bunch of monks in 3e, they are a lot of fun flavor wise but have mechanical issues.
All classes in 3e and 4e and 5e are designed to be roughly balanced in combat, able to participate significantly and have a role in combats. It is one of the great design features of 3e over older class designs in my opinion.
Mechanically monks and rogues do not quite live up to that design goal in 3e.
Monks are supposed to be mobile martial artist combatants, not just OK if forced into a combat role. The 0e/1e class was based on Remo Williams the Destroyer, a kick-butt dominant combatant, although the class design was really hurt by being mechanically alt thieves and having low hp. In 2e they got kind of an upgrade when they showed up as alt clerics so better attacks and hp, which is pretty much the chassis from then on for later editions (attack bonus and hp comparable to a cleric in 2e, 3e, 4e, and 5e).
Monks are mobile with a fortitude save stunning blow and have good defensive saves. They are decent at getting to the back lines and punching out a glass cannon wizard with a weak fortitude save and low AC. They are also better than others at fighting if they have nothing to work with, such as in a slave/prison escape or at a fancy dinner party, or if awakened by a combat at night when nobody has armor.
They are not really holding their own in most straight up combats however.
3/4 BAB means their attacks are not competitive with the warriors and they do not have spells for buffs or alternative attacks like clerics, druids, and bards do. Also they normally attack unarmed so no magic weapon buffs that others get. So things with decent AC are a big problem, their flurry of blows often turns into a flurry of misses when they can pull off a full attack. This means their offense mechanically is fairly limited. Their big offensive threat is a lot of attacks against a low AC opponent when they can get off a full attack, a con save rider a few times a day, and sometimes getting to the right position at the cost of their multiple attacks. Fairly niche. Against normal opponents they miss a bunch and do not do a lot of damage and a lot of things have good fortitude saves.
Their unarmored but add in wisdom AC means they are about the rogue's level of AC at the lower levels, it takes a lot for them to get a decent AC for melee (usually when they can double up on stat enhancing items) which means they are a bit vulnerable in straight default attack combat. Then they have fewer hp than rangers and barbarians so they are squishier melee combatants. In 4e squishy mobile direct combatant could fit a striker role, but that requires higher offense capability than baseline for most classes which 3e monks do not have. They have good saves and evasion and fall protection and such, but in direct combat they are more vulnerable than most warrior classes.
I am a big fan of the Trailblazer phantom attack bonuses so their attacks are competitive, and the Pathfinder hybrid monk-fighter brawlers who get a full BAB.
