Before revision:  Once the propensity score is available, one can use it to estimate a variety of causal effects in a unified setting. Despite this appeal, a main practical difficulty arises because the propensity score is usually unknown, has to be estimated, and extreme propensity score estimates  can lead to distorted inference procedures .
After revision:  However, it is well documented that under slight model misspecifications, propensity score estimates can lead to distorted inference procedures .