Buying the gunblade has a great importance in this build, without it is just a regular AD build, but with it it gives you the sustain you really need, behind it comes that unique active that is great to finish off enemies, it might seem odd, but master yi is really squishy, and as you go into late game, easely defeatable, with hextech gunblade, you can finnally play master yi in higher levels.

YI AD is great for low level game, but easily defeated when you are playing against higher level summoners
advantages:
-brutal damage
-brutal atack speed
-brutal movement speed
disadvantages:
-squishy, when your opponents buy armor, you quicly become useless
-impossible to play in high levels

YI AP
advantages:
-giant sustain
-high bursts of damage with alpha strike
Disadvantages:
-useless in destroying towers
- if you ran out of mana it becomes utterly useless

Hybrid
advantages:
-good sustain
-decent damage
-increased damage with alpha strike, that means armor is useless
-if you're skilled, it is easily playable in higher levels
disadvantages:
-takes less damage than yi AD
-has less sustain than yi AP