There's a thread on Airliners.net about Allegiant's reliability issues with the 757. The concensus seems to be that they have a very small fleet operating long-ish sectors, so they don't have a/c backups everywhere to get over problems as they occur. Also, they're finding ETOPS routes to Hawaii dictate a far higher level of serviceability than mainland USA op's.
It's not necessarily the aircraft that's the problem but how they're using them.