It seems so simple. Just brew a beer, drop a few spices in it, tweak and push "brew."
Beer turns out to be quite a complicated animal; spiced and other flavored beers are especially so. Even as someone who has been experimenting with spiced beers since the late 1980s, I am constantly surprised by the twists and turns that happen when you start adding specialty ingredients to a recipe.
I suppose I shouldn't be so shocked. It's difficult enough to design and brew a beer using the dozens of conventional malts and hops available. Far from being a straightforward additive process, the flavors and aromas also interact with each other, changing the flavor balance. Sweet malts balance the bitterness of hops. Roast grains add bitterness as well as roasty flavors. Carbonation mask hops. Yeast chooses its own favorite flavors to enhance or inhibit, and that varies by strain and temperature as well.
Now, add spices to that. At the end of the whole process, you want a balanced and layered beer to wind up in the glass. You have to worry about the nose, the initial payoff in the mouth, the length and quality of the finish and the lingering aftertaste. It should all tie together and create a singular impression, with no overpowering flavors or rough edges. That's just where you start.
Every spice has a personality. Some, like wintergreen or cumin, are bullies, elbowing their way to the front every time, even when used in miniscule quantities. Some, like vanilla (is there really anything like vanilla?) are peacemakers, throwing their arms around everything and bringing disparate elements together and rounding off rough edges at a cost of sometimes blurring things a bit. Black pepper and others heighten the intensity of the whole mix and add a certain crispness of their own. There are a lot of interactions in ways that are often unexpected and even hard to wrap your head around.
If you know anything about hops or grapes, you know that terroir matters. It's the same for spices. For each flavoring ingredient, the variety, growing location and conditions, harvesting, storage and usage changes the flavor, often quite dramatically, as the mix of dozens of important essential oils and other flavor chemicals changes in response to these conditions. Working with a spice like coriander, you really have to become a bit of an expert on it to figure out what makes it tick and which specific one—piney, lemony, orangey, eucalyptus-like—will be best suited for any given purpose.
It's a hard process, but an engaging one. It takes batch after batch on a small scale to get a prototype working properly, and the process has to begin all over in full production volumes. And when you finally get the combination right and add that capstone flavor that brings it all together, it really is quite a thrill to sip and savor the results.