I can see that. I do think that Family Guy's gags, however, miss roughly half the time, and of those misses, a pretty large number of them are extremely flat or cringe-inducing. So when the humor fails, the show doesn't have story or characterization to fall back on, leading to a mediocre, forgettable episode. I can see how some can prefer American Dad's more story-driven approach, since the MacFarlane shows are already so hit-or-miss with the humor.
On the plus side, even the worst Family Guy episodes have a couple isolated moments that are genuinely funny. They just need to know what to cut and what to keep in. A gag-driven show is fine (Rocko's Modern Life and Seinfeld are two of my favorite shows, and they both pulled that off really well), but they need to know when to trim the fat. That combined with the show's general smugness can be a turn-off.