Thunder is lacking core debuffs, especially right now. Look at Slashley's team from the video he just posted. He has a pretty good team, yet he's still only fielding -25% ATK and -30% DEF, and he's relying on Mordred's -10% class B debuff. On top of that, he has no healing, so if that Ragnarok boss came at the end of a series of trash and no healing potions dropped, he'd be in serious trouble. Thunder has to compensate for this fragility with overwhelming damage, enough to end the fight quickly before attrition sets in. That means it won't do well without multiple high-powered SSRs in content that's harder than Advent Ragnaroks. SRs just won't cut it, especially for Burst damage.
Light is fine for now (so long as you have Diana or Amaterasu), as it has some good SRs that can fill essential utility skills that players would otherwise need the EX slot / Soul for. But Light isn't going to scale very well when more difficult content comes along, especially prior to the 2nd Miracle ticket and Sol's Awakening. Accessory Quests are coming soon (probably in the next month), so you'll get a chance to see for yourself. IMO, relying on Sniper Shot being slotted at all times is a pretty fundamental weakness. That goes for Thunder too.