The autopilot only has control of the ailerons, remember. Coordinated flight depends on rudder/tail control and authority. For most experimental aircraft, once underway at a fast enough clip, flying with just the ailerons works just fine, which is why an aileron-only AP control for heading/track works in almost all cases.
If the aircraft can be banked 10-15 degrees and hold a constant heading, then you may have been standing on the rudder, severely out of rudder trim, or the airplane could be out of rig. Put another way - the AP can only do what you can do with the stick in your hand. If you can bank the airplane 10-15 degrees and hold a heading with the rudder neutral, there's something more fundamentally up with the airplane's control qualities.
