I enjoy Hackerrank very much. Although I've been away for awhile from their site. The coding platform was language-choice-flexible and I did a number of challenges in Clojure as a learning experience - very similar to the challenges from your old company, although I only played with round 1 of the security challenge.

I am not a fan of the mathematical challenges on Hackerrank, mainly because I haven't done much interesting math in years and if I get all ambitious on math stuff, I'm going to dive into functional analysis and stats modeling and probably not from a coding viewpoint. Algorithmic challenges are much more engaging to me.

Hackerrank has a very interesting option for hiring companies to sponsor a challenge or set of challenges. Based on what I read here, if I was a hiring company I would be doubling down on this approach (eg coding challenges as a feed for potential hires like you guys were doing as well). Heck, companies would probably pay five to six figures just for a curated list of candidates from these sites, right?

One specific gripe - sometimes I had the vague feeling that the programmatic challenges encourage non-idiomatic language usage. I did a very fun challenge that involved range-minimum-queries there and to ultimately to have runtime under the JVM cutoff and pass all the testcases, I kind of optimized away from the way Clojure normally looks like. I think my point is code that works well for a programming challenge may not look much like code you would want in production.

I can probably spout off more opinions but this is probably already more than you were looking for for someone who didn't use the site for actually finding a job.