If the target of Yuffie's last ability is illegal as the ability tries to resolve, it won't resolve and none of its effects will happen. You won't attach any Equipment to Yuffie.

The ninjutsu ability can be activated only after blockers have been declared. Before then, attacking creatures are neither blocked nor unblocked.

The ninjutsu ability can be activated during the declare blockers step, combat damage step, or end of combat step. If you wait until after the declare blockers step, because all combat damage is dealt at once, Yuffie won't normally deal combat damage.

As you activate Yuffie's ninjutsu ability, you reveal Yuffie from your hand and return the attacking creature. Yuffie isn't put onto the battlefield until the ability resolves. If Yuffie leaves your hand before then, it won't enter at all.

You can't use Yuffie's last ability to try to attach an Equipment to Yuffie if that Equipment can't legally be attached to Yuffie.

If you lose control of Yuffie or it leaves the battlefield before its last ability resolves, you won't gain control of the target noncreature artifact.

Yuffie enters attacking the same player, planeswalker, or battle the returned creature was attacking. This is a rule specific to ninjutsu; in other cases, when a creature is put onto the battlefield attacking, that creature's controller chooses which player, planeswalker, or battle it's attacking.

If at least one creature in combat has first strike or double strike, you can activate Yuffie's ninjutsu ability during the first-strike combat damage step. In that case, Yuffie will deal combat damage during the regular combat damage step, even if it has first strike.

Although Yuffie enters attacking when its ninjutsu ability resolves, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).