Is Quark as secure as SHA 256?
The biggest problem with Quark (or any other "memory hard" algorithm) as a hashing algorithm is that verification is synchronous. So in order to verify things you have to do a lot of work, compared to the very small amount of work required to verify SHA256. This could lead to a DoS attack where an attacker sends you a whole lot of bad blocks and makes you tie up memory and CPU trying to verify them.