They are no where near as reliable as the 2.0.
They NEED bigger head studs to prevent head gasket failure, the block liners are so much thinner than the 2.0 blocks making them flex a lot more, so if going big power they normally need replacement liners. They have chocolate pistons and ring lands.
The only time a 2.5 block from an STi is reliable is when it has been rebuilt with decent pistons, larger head studs, thicket head gaskets and a good tune.