Rav Yitzchok of Stutchin zy”a (quoted in Sefer Chochmah M’ayin, page 60) related that the Magid of Kozhnitz zy”a said that it is not possible to say anything good about Korach because he caused pain to Moshe Rabenu. It is also not possible to say anything bad about him because he is the ancestor of the Chozeh of Lublin zy”a.
Rav Yitzchok added that we see that the Magid indeed has no chiddushim on Parshas Korach in his Sefer Avodas Yisroel.