Terrorists adapt. Subtle can be a lot more effective then flamboyant, and they can feed off each other. It also isn't new.

Car bomb goes off or a fire starts. 20, 30 minutes later, there is a second device that catches the responders. Really sneaky buggers would put out a couple of decoys to further delay any second line of responders. (Been done, many times.)

Shelters are public knowledge. Toss a couple of unguided rockets into a town, everyone takes shelter. The truck that has been turned into a two ton claymore is parked outside the largest shelter, and goes off by remote two or three minutes after the all clear sounds.

Have a few guys shoot at on side of a perimeter. While everyone is looking, you send a sapper or snatch team in on one of the other sides, or you've already put infiltrators in, who execute the real mission. (Been done, many times. Like at the original Jericho.)

Subtle could be done. And enough people are on edge enough that it could do more damage than the initial flashy attack.