Before even looking into RocksDB we tried TokuMX and even got pretty well aquatinted with their dev team. We ran into the same issues when testing Wired Tiger. Neither could handle millions of mongo collections. Since we were involved in the RockDB storage engine from the beginning, we made sure the implementation could handle that many collections.

This isn't my post and I don't work at Baqend, but I will say that the author comes off as a presumptuous asshat.