Going into any mission or raid with a level deficit is just bad news . Take the level 30 raid , if your a level 29 you do 34% less damage to enemies and take 34% more damage from enemies .
Missions are one thing , it's easy to backpack a level 26 though the nightfall but now to the level 30 raid take a hole group of maxed out level 29's and it's just a massacre .
Hope this helps !