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Is it just 90 or 360?

Are the weapons hull mounted? If so, 45 degrees total

They're definitely Hull Mounted. They have no traverse at all. So the standard 45 applies as nosferatu says.

Since they cannot move slower than 18 inches doesn't that make them really bad?

It's the same arc as the Ork fliers.
And the Imperial Guard.
And Eldar, Dark Eldar, Dark Angles....

Most fliers have their weapons Hull Mounted. It doesn't make them all "really bad" but means you have to plan their use more carefully.

Oh I didn't know that sorry... in that case then, Heldrakes are super overpowered in comparison?

In terms of firing arc yes. However, they lack the number or choice of weapons you normally see on flyers.