I'm still a bit overwhelmed by all the new features in this game, compared to XIII.
I have some problems with the party paradigms with the addition of the monsters: it's basically like having all party members available in XIII not so many hours into the game, when you had the chance to try them out, but instead all at once after a bit.
The Crystarium is really different in this one and you can make lots of choices. So far I leveled mainly COM for Noel (with just a couple of levels in SEN and RAV) and RAV for Serah. I had two expansions for both, and I choose COM bonus for Noel, RAV bonus for Serah, and +20 accessory for both.
Noticed a bit of a difficulty spike in chapter 3, both first and second part.
Am I doing it wrong? Should I level up roles more evenly? Does it matter?
It doesn't matter in terms of being able to beat the game, but if you care about reaching the max possible strength for Noel, or the max possible magic for Serah, then you've done it wrong.
The larger spheres in the Crystarium give you a bonus depending on which role you used to take that node. For example, if you leveled SEN on a large node, you'd gain additional HP bonus to your character. COM gets a +2 Strength Bonus, RAV gives you a +2 Magic Bonus, SAB gives you +2 Magic if you level from an Even number to an Odd Number. Example: Level 10 SAB, you take a big node and go to level 11, you gain +2 Magic. If you level from an Odd to Even number, Level 11 SAB to level 12, for example, you'll gain +2 Strength.
So far I leveled mainly COM for Noel (with just a couple of levels in SEN and RAV) and RAV for Serah.
Even if you don't care about stats, I'd recommend against continuing this. Aside from the stat bonus from leveling a role on a large node the only thing leveling a role does is unlocks abilities. It is much more beneficial to level a variety of roles to give you more options in battles than to just stick to one or two roles until they reach their cap before leveling other roles.