Troubleshooting Substance Painter Problems
Ngons are problematic due to the way different DCCs triangulates them. It doesn't always cause problems, but in this case substance painter fails to triangulate the ngon.
Solution: Triangulate any ngons before exporting to substance painter.
UV coordinate overflow
By default, unwrapped uv islands will fill up the entire uv space in blender. UV islands with vertex touching the UV border wraps around to the other side. Seems to be a bug.
Solution: Scale the uv islands so the edge doesn't touch the border.
World Space Normal Bakes Checker Pattern
Specifically for stacked UV islands.
Solution: Reason of why this is happening is not exactly known. Stacking outward facing faces together solves it.