Nerf / Ninja Gaiden

• The Flying Swallow in the Xbox game was toned down in Black because you could spam it on pretty much everything with 100% success. Black featured mooks that were specifically designed to punish you for overabusing it, i.e. block the attack entirely and counter it. You definitely didn't want to be caught using that on the Advanced MSAT soldiers.
• The counter system: in the first game, counters were fairly easy to execute with little effort in timing of an enemy's attack. Coupled with a powerful weapon, such as the Dabilahro, counters turn into OneHitKOs. The sequels required stricter timing and plenty of enemies' attacks are combo-driven.
• The Spin Attack from the first NES Ninja Gaiden was removed from the sequels. The Invincible Fire Wheel, an equippable (albeit expensive) Ninja Art was turned into a limited, single-use powerup for The Ancient Ship of Doom.
• The Art of the Fire Wheel is further nerfed for the Xbox Ninja Gaiden compared to the NES days. It creates a flaming shield around Ryu, but it knocks away most enemies on contact which severely limits its usefulness (continuous damage is impossible and it knocks them out of your melee weapon's effective range). You are also far from invincible. It has been un-nerfed in Razor's Edge, as Momiji's Ninpo, although her ki gauge takes a long time to fill.
• Incendiary Kunai in II systematically dismembered enemies the first time and killed them with a second one. In Sigma 2, those used by Ayane are quite effective in normal difficulty, but are about as useful as shuriken at higher difficulties. Sigma 2 also removes the ability to charge arrows. Then again, since projectiles are rendered infinite in use, keeping them as powerful would've turned them into Game Breakers.
• Ninja Gaiden III indirectly nerfed the Izuna Drop: it's still an instant kill technique, but tougher human-sized mooks have to be weakened before you can lift them up, so it's not quite as overabused as in previous games.