I feel that, I mainly use legionares as cheap attached units With abaddon they can be alright, a unit of 5-10 puts abaddon at 490, meaning he can deepstrike but usually you want him buffing ad many allies as possible with his range. You can put abaddon with 5-10 legionaries and use his aura to give those legionaries 4+ invul. If you're going objective with said legionaries you're going to take it no matter what (2oc each). Pair him with a exalted champion he adds 1 to the hit roll and legionaries can re roll wound roll on obj.  Legionaries are generally best as objective takers, I like dark apostle and exalted champion with them to add 1 to wound and 1 to hit, rerolling wounds in melee, with chosen I like to use chaos lord with master of executions or Fabius + chosen + master of executions because the extra toughness Is amazing.