...little context, please?
Ok, so straight Monks (do not forget that a two-level Monk-dip is pretty darn good for many builds, and Tashalatora-, Sacred Fist- and Druid-based builds all make fine "Monks") aren't good at their own role...their own role being?
They aren't exceptional tanks due to lowish HD, medium BAB, multi-attribute dependency (and thus comparably lower combat stats than melee monsters; this also hurts their supposed strengths in Grapple, Tripping & other combat maneuvers, along with Stunning Fist; all of those heavily reward straightforward dedication to a single stat over all else, and a Monk really can't pull that off), the fact that you can't combine their movement speed with Flurry (Flurry requires full attack, movement allows only one) and lack of weapon proficiencies (unarmed strikes getting decent dice later on, but lacking in special abilities and enhancing them costs a fuckton; oh, and no reach, no AoO-builds). Flurry is needed for them to do decent damage forcing them to ignore their speed boost in combat.
They aren't exceptional scouts due to lacking Trapfinding and having relatively low skill points and being unable to afford decent Int thanks to multi-attribute dependency (Hide/Move Silently/Tumble is all good, but if you don't have Trapfinding, scouting ahead in a hostile environment is like to get you killed).
They aren't exceptional mage killers (*chuckle*) because they really have nothing to especially threaten mages with. Just like every other warrior type, their movement is inferior to teleportation (once-per-day Dimension Door doesn't cut it), they have few if any ways to locate the mage and penetrate magical defenses (Mirror Image + Displacement + Blink: good luck hitting... Or Wall of Force) and they can't even reasonably use bows so their ability to act at range is infinitely diminished. Oh, and if they somehow manage to plop an Anti-Magic Field around themselves? They just gave up like 70% of their class features. Thanks to Greater Spell Penetration (in Core)/Assay Resistance (out of Core), their multi-attribute dependency, spells that ignore saves (even just good ol' Rays like Enervation/Scorching Ray/whatever, or Forcecages or something dumb), spells that trivialize touch AC (hello, True Strike!) and so on, all their magical defenses really add up to jack shit.
They aren't exceptional skirmishers due to not being able to Flurry with standard action and their speed bonus being enhancement thus, while probably being able to somewhat remain out of the harm's way with Spring Attack, not reducing the damage their allies take one bit and dealing negligible damage themselves. Indeed, this is the worst thing a Monk can do since it means the people who do the fighting are now taking all the beatdown while the Monk isn't contributing to the team's damage in any meaningful way either. In other words, the Monk isn't taking any hits and he isn't dealing any damage this way; thus he's as good as an empty slot in the party.
And overall, their class features kinda suck. Mostly, you can look at 'em like this:
-Flurry? That's nice! Now if only I were able to focus on one stat and have full BAB, I'd be doing a lot with my extra attacks on highest bonus!
-Improved Grapple/Trip/Stunning Fist/whatever? Nice! Now, if I only were able to focus on one stat and have full BAB, I could be landing these and winning the opposed checks!
-Speed boost? That's nice! Now, if I only could move and attack with my Flurry (which "almost" makes me equal to full BAB types), I could be doing something! Oh, and if this only stacked with magical speed boosts I'd actually be faster than the other classes.
-Unarmed Strikes? That's nice! Now, if I only got size increases or something so the damage dice would actually add up to something, and got 2x Power Attack returns and full BAB, this could add up to something!
-Ki Strikes? Nice, my unarmed strikes pretend to be weapons and get some minor abilities that almost replicate what my 1000gp weapon does! If only my WPL wasn't 100000...
-Slow Fall? So I get to replicate a 1st level spell by level 20? No? It only works next to walls? Well, almost replicate a 1st level spell!
-All this nice stuff, Abundant Step, Quivering Palm, Empty Body, I can replicate many kinds of spells poorly...once per DAY! Oh, make it Once per WEEK for that scary scary, broken Finger of Death With Save DC Derived Off Secondary Stat That Requires An Attack To Hit To Be Used.
-Oh, there's more? I get to replicate few more random low level spells? Cool. Oh, and Evasion? Yeah, nice, my Reflex-saves actually matter something! That's like...25k saved on the Ring.
-I get Spell Resistance? Just to ensure my team can't waste a Heal on me when I'm about to die? Cool!
Lack of synergy and multi-attribute dependency pretty much screw Monks up. Oh, and the good class features being limited to Very Few Uses Per Day. Seriously, if Monks had the ability to use Flurry whenever making an attack, if they got like Wis x uses of their now-daily abilities and the ability to use Dex for combat maneuvers, and Wis/Dex for damage, they'd be just fine. Grab Weapon Finesse/Intuitive Attack and they'd be able to go to town. As all those things are fucked up though, they don't. As I mentioned above, those multiclass builds easily sidestep these issues. Mono-classed Monks don't though.
Also, if this isn't on target, it's all because your question wasn't precise enough.