Good comparision. MeiliSearch's BucketSort and RAM consumption are the reasons I decided not to use MeiliSearch after testing it.
I just couldn't tweak ranking with BucketSort to get a desired relevancy. And memory usage was extremely high.
I didn't bother testing Manticore since there was no good open source client for C#, but seeing they released an official client I might test it to see how it compares to Solr.
Solr needs better documentation and community, but it's crazy fast, stable, and works well for what I do with search. It could also stand to have an improved UI to enable a better getting started experience and an easy means to index webpages out of the gate.
If anyone were to search for how to index a webpage with it, they'd get frustrated pretty quickly. I started a new version of Grub to address this, but I've found my motivation for working on it lacking: https://github.com/kordless/grub-2.0
A friend and I assembled a module for that repo called Aperture, which implements a scalable screenshot crawler that sends document into Solr. If anyone is interested, hit me up and I'll provide some instructions for using it!