Making a frame using 1"x4" wood. Exposed ends are not a concern. Which would be a stronger joint, a mitered corner with glue and 1" pins (2 from each side), or a butt joint with glue and pocket holes?
The second, by a substantial margin.
The claims of strength from purveyors of pocket-screw jigs have been tested by many woodworkers, including numerous pros (e.g. Bob Van Dyke of the Connecticut Valley School of Woodworking), and these tests often confirm that when used for a standard 90° joint between two boards strength can rival or exceed that of mortise-and-tenon joinery. No basic mitre joint reinforced with a few brads can match that.
Now that said, the mitre joint might be strong enough. So in the right context if you want to use mitres for appearance reasons — they make for a very neat corner, and there are no pocket-screw holes to hide or try to fill/plug in a way that looks good — then you can, without having to worry the thing is going to fall apart.