The main reason I do not think Warforged Monks are powerful is...they can't wear the Epic Frozen Tunic.
But why is this so important you ask? It's fairly simple actually: that tunic gives you +1 Ki on hit. Yes, there are two other items that give +1 Ki on hit as well, but both of those are necklaces, which means you cannot wear the Shintao set. And losing either the +2/+2 from Shintao or the +1 Ki/hit is very painful to Monk DPS.
Some people just say to use Fire stance to solve the Ki issue. However Fire stance is the worst possible stance past the early levels. You don't have the massive DPS that Wind stance gives you, the survivability that Earth stance gives you, or the better DCs that Water stance gives you. By trying to overcome the racial disability (not being able to wear the right robe), you end up losing even more DPS, just from your choice of race.
The healing amp issue is also important. Yes, you can wear Jidz-Tet'Ka and use Fire stance (and also spend 14 AP on healing amp) just to get slightly over what another race starts with base (.65 * 1.3 * 1.25 = 1.05625), but that again comes with another DPS loss, from not wearing the non-epic Claw set while leveling (+2 damage, takes up gloves + bracers), or the Epic Claw set while at cap (+4 damage and +30% healing amp).
Plus, the -2 WIS gives -1 DC to many Monk abilities. Yes, you can spend 2 AP to get Stunning Fist back up to normal, but that doesn't apply to Quivering Palm or Touch of Death.
Warforged won't make a completely worthless Monk. The problem is that it just doesn't mesh well with the class at the current time, and there are much stronger options to choose from (helf or human, even horc).