Angst? What Angst?: Averted. Chapter 27, volume II (Ranma 1/2 second arc), has the SI finally break down and start crying. He explains exactly why to Shampoo, and even she feels sorry for him. She even admits that she wouldn't be as strong as the SI is being, and would probably either give up or kill herself in his shoes.
Crazy Awesome: Only when the SI has been awake for essentially 25 hours with no sleep. The sleep deprivation causes him to go loopy.
Game Breaker: The part of Chapter 15 where he gets advice from Spacebattles has numerous forum goers encouraging him to be this.
Marty Stu: The author is actively trying to avert this trope.
For the most part, the knowledge, picking up useful equipment, and appearing near mostly benevolent characters are enough to make up for his purely human skills, but the Nanoha sequence illustrates how it can systematically go drastically wrong. After waking up next to Quattro, Jail easily calls his bluff. He then takes the rainbow crystal that the SI picked up from Zoicite in the Sailor Moon universe which was part of the bluff, and realizes how powerful it is. When the SI launches his retrieval plan, he does a fake use of Dark Schneider's Venom like he did in the Ah My Goddess universe, and ends up impaling Cinque in a wall. This results in a furious Tres into punching him out.