The accuracy and reliability of various editions of Midrashim, especially Midrash Rabbah, is not great. I have the few available volumes by Mechon Schecter, who have been putting out critical editions of Midrashim. However, most of the Midrash Rabbah volumes have not been released. Where can one buy an accurate and reliable edition of Midrash Rabbah?
Midrash Rabbah is a collection of disparate texts, so you shouldn't expect to find a single good all-encompassing edition. As far as I know, besides the Schechter Institutes editions you already mentioned, here are some of the critical editions available:
- Bereishit Rabbah: ed. Theodore–Albeck
- Shemot Rabbah: ed. Shinan (on chapters 1–14)
- Vayikra Rabbah: ed. Margaliot (2 volumes), ed. Milikowsky–Schlüter (online)
- Devarim Rabbah: ed. Lieberman
With the Shechter Institutes having already published Esther, Kohelet, and planning to publish Bemidbar, Ruth, Eikhah, and Shir HaShirim, you've essentially got your Midrash Rabbah covered. (I don't know of prior critical editions of those, nor of a critical edition of the rest of Shemot Rabbah).