I love this spell, and I think it outclasses most other high level combat spells for general use.
For Mystics, one less XP than either Shrivelling (5) or Azure Flame (5), gets you three less total damage and one less +skill value. You also get a downside that is both less likely to trigger on its own and is completely mitigateable by spending other actions first.
But the real upside here is choosing how much of that 9 damage to spend on every attack. No more wasting a three damage charge on an enemy with only one or two health. Charges are also only spent when you succeed, so bad luck with the chaos bag doesn't waste charges. You will get the full nine damage out of this every time.
Nearly every other mystic combat spell demands you combine it with some sort of chaos bag manipulation to get the best results, either to find or avoid specific tokens. Brand of Cthugha works at full effectiveness straight out of the box.
The one downside is that Brand of Cthugha does not get as much benefit out of charge reloading cards as its higher level peers. Each charge is only worth one damage, so you wouldn't waste time reloading it with a Recharge or Book of Shadows. This is the ultimate Mystic sidearm, not a god slayer, so plan accordingly.
But let's not forget that this card is dual class, too! Most Guardians aren't doing much with their arcane slots, so Brand of Cthugha slots in nicely as a sidearm that doesn't take up a hand slot. This works well for any Guardian who would rather not dedicate their body slot to a Bandolier to hold a sidearm. Tommy Muldoon and his Leather Coat come to mind, along with anyone running Flamethrower looking to save supplies while picking off runts.