Hi Kevin;
1) EAS 5,5 only supports SHA-1 certificates. Support for SHA-2 was added in EAS 6.1.
2) EAS 5.5 is very stable. My Canadian government clients still use this release (not many of those left these days however).
3) EAS 5.5 is much faster handling NVUO's because it deals with the C++ objects directly. The EAS 6.x architecture changed that and started treating NVUO's as Beans - hence interfacing through a Java wrapper, which made the execution much slower. 
HTH
Regards ... Chris