yeah but aang either ran or was getting washed by those enemies.
he didn't face evil spirits on the same level korra did
the entire group couldn't do anything against combustion man they just evaded and ran whenever they fought him and korra never fought Pl'i anyway
not to mention you say aang fought those types of enemies and I'm not saying he didn't but the scale was weaker. the combustion woman could control her explosions directions while the original guy couldn't and aang didn't fight evil spirits on the same level as korra
on average korra's enemies have been stronger