She loses flurry of blows but gains the ability to run 2+ "fighting styles" at once w/o even needing to follow the normal prerequisites. At UA 2 / Monk 3, I had her with Snake + Crane style in a world full of old-style guns. Guns within first-range increment use touch AC, which is devastating for most characters, but I had my monk's touch AC = 22, which beats many regular ACs at fifth level, and only taking a -1 when fighting defensively to achieve that. She also is the Attack of Opportunity goddess, pretty much anything creates an AoO and/or retaliatory attack if the attacker misses, so despite only doing average damage per round (1d6+4), with her Dex at a 16 I could get up to four add'l attacks per round if people attacked her and missed... plus any retaliatory attack from the Snake Fang feat.
You can get a third stance active at monk level 8 and there are different ways to approach it. I don't know if Panther Style is as effective, and it demands that enemies choose to attack you, but it's hilarious if you get all the feats. Along with all the AoOs you normally get for Crane + Snake, Panther Style means you get a free Swift action retaliation attack if you move through a threatened square and the guy attacks you. If you get the other two feats, you get Free action retaliation attacks (up to your Wis bonus which is a +4 for me) that you can make BEFORE the other guy's attack is made -- iow, you instigate a free attack by your enemy but can predict his attack so you strike FIRSt. (So if you kill him, he never gets to make the attack.) And meanwhile you are moving a normal move through threatened squares. The biggest flaws with this plan are (1) you want to have a good enough AC that you will not get hit by these attacks and (2) the enemy has to have an AoO to attack you with / chooses to attack you with the AoO. If they "get smart" and realize you will clobber them for attacking, they could just let you go through without attacking... but at least you'd get unmolested movement through their square(s).
But realistically, anyway, if I maxed out my attacks, I could get up to 4 retaliatory attacks from Panther in a round + up to 4 more AoOs from Snake/Crane defense + an immediate action attack (Snake Fang) + my regular attack. That's 10 unarmed attacks per round (1d6+4 = average 7) spread out across, likely, across eight opponents.
THEN... if she takes Vicious Stomp and uses Trip attacks as part of those free AoO attacks she can make ... Vicious Stomp allows you to attack someone as they fall, normally you can only AoO them when they stand back up. So you trip them, stomp them as they fall, then get ANOTHER attack on them next turn if they choose to stand up again (and if they don't, you just kick them some more). Just more opportunities for AoOs.
She might only be a monkeyan type, and not do mega-damage, but it's like watching those martial arts movies where she dances through the crowd triggering and dodging attacks while kicking, tripping, punching in retaliation without getting hit. It would have been fun to actually play it out against low-level gun wielders, firing wildly at her and probably hitting each other in the middle of a crowd.