"The biggest problem with asynchronous resets is that they are asynchronous, both at the assertion and at the de-assertion of the reset. The assertion is a non issue, the de-assertion is the issue. If the asynchronous reset is released at or near the active clock edge of a flip-flop, the output of the flip-flop could go metastable and thus the reset state of the ASIC could be lost."
Source: Page 18 , section 5.4 of http://www.sunburst-design.com/papers/CummingsSNUG2003Boston_Resets.pdf