it felt really good to me with ring of focused flame, scion of flame. with those two things added in, it's pretty much an S-tier weapon (better than other summoned weapons imo, with possible exception of woedica's fists), any martial would love to have that weapon. with some wizard buffs (esp deleterious alacrity of motion, and arcane reflection is actually relevant bc you're so constantly close enough you'll actually get targeted by spells) it felt very satisfying to basically zip around, untouchable, and burn everything. it's not an unfair build, but still a strong one.
spiritshift was a nice fallback against fire resistant/immune/absorbing enemies.
edit: wall of draining is always going to be unfair, but when you do get wall of draining you can extend even small duration wizard buffs (eldritch aim) and cat's flurry from druid (+33% action speed).
edit 2: all summoning weapons kind of hit a low-point mid-game because they're not scaling as fast as weapons as you typically find, so depending on when you tried it, it might not have felt as good. but +10 acc and +1 PEN really help firebrand continue to feel very strong even it's stuck at fine or exceptional scaling way too long.
edit 3: this is a kind of build that really benefits from AI scripting. there's just so many fast-cast buffs you end up wanting to cast from wizard that I had an AI script just for this build to go through: deleterious, reflection, ironskin, fireshield, infuse with vital essence, etc. and because they all have no recovery, there's no real action economy issue to just spamming these all at start of fight. arcane dampener from bad guys suck a lot but deleterious (or fleet feet early on) + fast runner (normally not available to wizard but you get from druid) + possibly some boots makes you so dang fast you can just run away from most fights until the enemy wizards waste their dampener on someone else.
edit 4: oh yeah, as boeroer mentioned, i completely forgot about the interaction with phantom. honestly i don't think i used that for my build (it's been a while), i just completely forgot about it for my build, was too focused on zipping around with firebrand, but it definitely would amp up the general power and utility of the multiclass. i forget if spiritshift also copies over to phantom.
warning though that i just remembered (and i noted this in my guide) - sorcerer has a really weird bug, where if you switch grimoires while spiritshifted you lose all the spells that come from grimoires. you can get them back after combat if you're not spiritshifted and you switch grimoires again, but it's a pretty nasty bug and if you do go down a sorcerer route you should really make sure to be careful about it.