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.
We had issues with the MMAPv1 storage engine from the beginning, mostly due to it's lack of document level locking and storage bloat.
One of the biggest benefits from being acquired by Facebook was working with the RocksDB team to come up with MongoDB +RocksDB, but this was also happening around the time when Parse was starting to wind down.
Being the only company running this new storage engine freaked us the fuck out. So we tried to blog and give talks as much as possible to get other folks interested and willing to test it out.
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.