He's no mobility, though amazing base MS. Anything with hard Engage eats him if he cannot stun them and since his stun requires set up there's basically a guaranteed second of them doing something to you before you can stop them if your E is not available.
His CC is conditional and unreliable. My names Xerath, I don't have to set up anything to Stun, and I've got longer range.
His ult is totally unreliable and can be completely canceled by statis / untargetable instead of merely bouncing off them doing nothing and eating a charge.
There is no 'Win' condition to his combo. You have to lose out on something by setting up blaze for the rest of your kit. Want to make sure your Ult prioritizes champs AoE? Then you burn W without 25% Amp, or E after a W or Q, meaning loss of stun or Amp. The only real combo that allows you to utilize your blaze bonuses to its fullest effect is opening with your Ult, but unless you're in a position where there are no possible bounces except champions, its dangerously unreliable.
To put it shortly, Brands kit is super fun, and super powerful, but hes punished by his own mechanics and the payout of this drawback only rarely makes him worth it unless you're someone who just loves his kit / theme. Brand definitely has his upsides, its just that hes counterable, has to work against his own kit, and can be painfully unreliable.
(I have probably 1,000 Brand games as a Brand main since the beginning of S2.)