Archers do not get backstab, based on the attack scripts that Bioware posted on the Wiki. But all rogues do get increased critical hit chance when flanking, and rogues also get automatic criticals when attacking from stealth. In addition, cunning seems to add to armor penetration, and with the lethality talent you get to use cunning for bow damage (normally the part that comes from strength) and it also increases your crit chance. Finally, taking assassin further increases your crit chance and gives you at least one good talent (mark of death).
The big advantages that warriors get are the ability to wear heavy armor without fatigue penalties, ability to switch to sword and shield style if they need more defense, and of course more stamina for spamming archery talents.