It's not that they are bad, it's just that other means have been determined to be better.
Wet/dry units are fantastic at creating oxygenation for a tank, and in breaking down ammonia>>>nitrates. But they do not remove nitrates. And the bioballs serve as a trap for detritus.
Now, we lean more toward an empty sump with a big protein skimmer, plus a refugium. The skimmer removes the bad stuff before it even breaks down, while a refugium and its macroalgae eliminates the nitrates.
I have read a few experts who still say that wet/drys are a good idea for very heavy bioload tanks (lots of big predators), because of their spectacular ability to break down ammonia. I'm curious if anyone has put a wet/dry and a big refugium together on the same tank. That might be a great combination.