Well, there's a few considerations:
1. It only works on a corpse. So you're reducing the difficulty of raising their body from a level 7 spell to level 5, and still need someone with the *Raise Dead* spell (*Revivify* has a time limit shorter than the cast time of *Mending*).
2. It's not doing *everything* that *Regenerate* can do, especially since the bulk of *Regenerate*'s power comes from its high hitpoint recovery.
3. Failure (which will be common on a high DC) carries its own potential consequences (what if reseparating the limb causes it to fall into multiple pieces?).