The drawback is that you're using Wrath on extra damage instead of on defense (Inner Beast). You have to know when you can get away with that and when you shouldn't. If it's a fight where you don't need the extra IB(s) provided by Infuriate, you can use Infuriate to get around that problem. If you need every IB you can get your hands on, you're obviously going to want to avoid Unchained in general. The rest of the time, it's a question of how soon you'll need IB and whether you've got Infuriate available.

For most fights, it's safe to use Unchained/Berserk early on. There usually isn't enough damage early on to worry about, and if there is, you can cover with Vengeance and probably have that ready again by the time you need it. Kitru's opening sequence leads to a pretty hefty threat lead, which lets you use SE/SP more often.

And if you are using Unchained on cooldown, it's best to save Berserk and use it at the same time rather than using Berserk on CD; the total damage is greater that way.