They advertise themselves as:
" The Slickest WordPress Stack
Ever Imagined.
Choose any datacenter in the world. We’ll setup a dedicated SlickStack VPS server for you and manage it to perfection (24/7). Lightning-fast loading speed, 100% free SSL certificates, and dozens of technical SEO features are included in every hosting plan…"
Which means they are in competition with other SEO providers. Which means their opinions are probably not entirely impartial.
I agree which is why I said I take their opinions with a pinch of salt. However, the fact remains that I can actually relate on a personal level to some of the comments in the LittleBizzy “scam alert”.
Also, there’s also this lengthy first-person review which concludes,
In my honest opinion, I have used this plugin for 3 or more months on two sites. Both the sites had problems with wordpress dashboard speed loading. It took lot of time for any operations to complete.
The Yoast Free version is better and tested well. It is more authoritative and well-used by many websites. The developers (mythemeshop) are simply making users the scapegoats in their testing cycle. See note 2 below.
It should have been tested thoroughly. I think in future, it may behave properly. This plugin coming from “marketing crazy” developers is not intended for live usage.
Note 2: This is what was so terrible about WPMU-DEV… lots of bugs, no apparent testing, push the code anyway and hope for the best.
Thanks to everybody bringing this information to the table. We definitely need to be scrupulous on these things - I’m personally going to keep Rank Math on the couple of builds I’ve deployed it to and see if there’s any problems ongoing - unfortunately they were small sites with small numbers of visitors anyway so I’m not sure if they’ll make for a good before-after comparison.
I think if there’s any unethical practices that the community are against alla fake-reviews and spam etc we definitely need to hold the plugin authors responsible and this should be reflected by whatever plugin repository we end up with (I’m side-tracking here though). I’m personally yet to receive any email from the author which is a sign that at least maybe they take the review feedback seriously?
@1stepforward Please take a look at https://petitions.classicpress.net and search for these issues or add a new one (I would use 2 separate petitions for these requests). This is currently our best tool to prioritize what gets included in future versions based on the community’s wishes.
Petitions that are specific and well-scoped (i.e. “allow editing page title and meta description”) are much easier to implement (for example “add SEO features” would quickly become a mammoth task because it’s not clear what exactly should be added).
Also, around the same time WP 5.0 came out and I switched to CP, my hits and impressions dropped off a cliff. I can’t help but wonder if something has been built in to recent versions of Yoast to conflict with non-Gutenberg sites.
Aside from a minor issue which I was able to fix, I haven’t had much trouble with Rank Math. Also, that blacklist includes respected biggies such as Wordfence and Autoptimize. Jetpack is in there, too.
Bit of an update:
I did start getting a couple of spam-like emails (starting from last week from the looks of it - though my junk mail might have emptied before that) - though I haven’t received any further ones on unsubscribing.
In terms of website performance - on the two low traffic sites I was using it with the performance seems to have remained much the same as when they were on Yoast. Though they weren’t getting loads of clicks anyway. I’m happy to look at alternatives, especially if certain features like Site Maps and Meta Editing end up in core anyway but for the time being I’m happy with the interface and the features RM bundles.
The latest Rankmath update 1.0.36.2 totally breaks the page editor with Classicpress - just shows blank page.
Yes, as of version 1.0.36, Rank Math has introduced support for Gutenberg. See this post.
As you will see, Classic SEO has moved on considerably in the past 7 months or so. If you’re using ClassicPress, then you should definitely consider switching to Classic SEO which is built specifically for CP. It is now pretty stable and a few people are using it on live sites although you might want to try it on a test site first if you’re able.
You can import settings from Rank Math so you shouldn’t lose anything when making the switch.
How do you find classic SEO? I did a search inn the “add plugin” and it did not show up. Nor could I find it in Google
He “found” it by writing it himself. 
@MrLucky to be precise, Classic SEO is a fork of RankMath.
When RankMath showed the intention of not supporting CP a bunch of people here (thankfully) had the idea of forking it.
And not only that… They even made the fork better than the original.
As for ClassicPress plugins, you can find them on GitHub or on the dev site since they aren’t on WP repo.
ClassicPress v. 2 will sport a plugin directory however, so it is going to be easy to find the plugins once the directory is in place.
Some news
From that discussion:
We will support Classic Press at least until 2022. After that, we will gauge the user demand and decide accordingly.
OK, that’s the first time they’ve ever publicly said that they support ClassicPress. 
I guess that leaves some decisions to make. According to the recent poll I did (which is still open btw), only 7 people said they were using Classic SEO.
If there isn’t the demand for it, particularly now that RM have said they’ll be supporting CP for at least another couple of years, is it worth my continuing with Classic SEO?
Or, to put it another way, are YOU still interested in using Classic SEO or would you rather use Rank Math?
Based on my limited testing of both, I thought Classic SEO was a better option…and less invasive. I’m actually still using Yoast on this end for awhile… Still, there is definitely value in continuing with Classic SEO… I’m sure it will be adopted by users as a ClassicPress-first solution over the others…and it would be an easy plugin to monetize later with premium extras. Rank Math has been a bit wishy-washy in declaring support for ClassicPress, which doesn’t give me a ton of confidence. If I were you, I’d stick with it!
Rank Math may have said they will support CP, but if they release updates that cause blank admin screens in CP (as they recently did), it suggests they’re not testing updates in CP before releasing them.
It’s one thing to say you support CP, and another to follow through.
Well, as I’ve openly admitted in the past (see above), I’ve never been the biggest fan of MyThemeShop, the company behind RM and I definitely don’t like how you need to create an account to use it.
I agree entirely and personally, I’d take what they say with a huge pinch of salt especially as this is the first time they’ve said they support CP. I’m guessing what they really mean is that they’ll continue to support WP 4.9, which is totally not the same as supporting CP. Or, perhaps they’re getting confused with Classic Editor?
But anyway, I’d be more than happy to continue developing Classic SEO if demand is there. To my mind, having to choose between a plugin which might still work with CP in a few months and one that is written specifically for CP is a no-brainer. But I’m not the one that needs convincing. 