Masamune's observations are mine too. In MTW the exploit was overfixed because of the very cheap border forts. In STW Shinobis are indeed overpowered and underpriced. It would have suffice to raise their training cost (perhaps 300 to 400 koku) to fix the problem (then 2 shonibis=1 border fort and not 8shinobis=1border fort that renders the bf obsolete in the current price).