The biggest issue with an improper Pardon is going to be standing. There aren't really that many Pardon cases out there, and the ones I can think of all involved the person receiving the Pardon going to Court. It's sort of unclear who has standing to sue if a Pardon is given.
Mueller's team *should* have standing, I think? Also Congress in their role as oversight on the Executive. I keep meaning to go look this up, and I never get around to it.
On the plus side, the Courts have repeatedly ruled that Presidential powers exercised for the personal benefit of the President aren't owed the same deference as actions taken for the good of the country.