Morganna is harder to play in general. Puddle is harder to use than Lux's AoE, ult requires melee contact as a mage, Skill-Shot is slightly harder to land. Still pretty great support caster.
If you are interested in playing the game past level 20, fill your empty runes with Tier 1s, then save for Tier 3s after you get Nocturne.
Tier 3s aren't so significant you literally can't play if you don't have them, but they're still a really huge comfort zone and they're so expensive you want 1-2 default pages that serve almost any hero.
HP Quints, Armor Pen Reds/Magic Pen Reds, MP-5/Dodge/Armor Yellows, Magic Resist/Cooldown Reduction Blues. Pick 1-2 from each color and you can cover almost every hero.
You could also do what I'm doing ATM. HP Quints, MR Reds, MR Blues, Armor Yellows. Fits literally every hero and favors the current defensive meta.
But yeah long story short Tier 3s are expensive as fuck. If you get bored force yourself to pick up every 450 hero and learn them a bit.