I just finished up playing an Aasimar Ninja/Vile Chemist short sword user, and leaned heavily on dark finishers. My goal was to combine the increased poison vulnerability from
Ninja Poison with Vile Chemist poison weapon damage, and it worked like a charm. The poison damage, including against otherwise poison-immune critters really cranked up nicely. Note, however, this was not a top-DPS build, even with the poison. I planned on leveraging the crowd control abilities dark monks have to make up for that, and that plan worked like a charm.
It was a wisdom build, but I did not use falconry attacks, mostly because I needed to get an Alchemist life out of the way and wanted to try out a poison-based melee fighter. I used Pain Touch and Freezing the Lifeblood all the time, and was very happy with them. I had a lot of points in WIS and they landed with ease through R5, including champs and even the less powerful red names with reasonable consistency.
I reserved Touch of Despair-Ninjutsu attacks for two occasions: when there were a LOT of mobs that I was going to deal with, Wave of Despair (WoD) was a nice way to debuff all of them at once; second, Poisoned Soul helped against tough bosses that were otherwise immune to other finishers. It seemed to be harder to land - I think it had a worse DC than other finishers. WoD is important because it affects DCs, and most finishers and special attacks allow saves. The alchemist/ninja combo allowed me to do a lot of debuffing. Imposing a -10 on fortitude saves on what I was fighting was not unusual, and even more at times. (Ninja attacks and finishers, along with VC attacks, rely on fortitude saves. I also used dual
Envenomed Blades, which is - you guessed it - a fortitude save.)
The complaint that they take a long time to charge up is right, but if you have good Concentration score, you can pre-charge, so to speak, by activating it before going into combat, and therefore be able to use it immediately. I do that with WoD a lot. You don't actually have to hit something in order to get credit for the three initial strikes.
Speaking of bad DCs, Touch of Death is nearly worthless. For reasons that elude me, it has a terrible DC. I think it's because it does not include stunning bonuses. So unless your WIS is at god-like levels, don't bother with it. Next, I hardly ever see Poison Exploit proc. From what I understand, Deadly Exploits is broken, so don't use it. But near as I can tell, Poison Exploit is mostly useless too.
I hope that some day the devs see fit to repair this badly damaged tree, but I have to say, I had a great time combining it (t5!) with Vile Chemist (t3). I went GoF in epic and never looked back. As for playing a pure monk and going all in with Ninja? NOT a good idea.
Anyway, all of this to say that you can put together a dark finisher monk, but, and it's a big but, I cannot lie (that's right, we can all use a little Sir Mix-a-Lot), it can require some creativity to make it fun and viable. You don't have to go the way I did, using alchemist, but as I said, I would not do it as a pure monk. The tree is too broken for that.