Beware of HackerRank
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I agree with the author on some cases; Frequently, sites like hackerrank have a specific case that causes Java Arrays.sort() to go O(n^2) and time out. Those feel dumb; there are many real life situations where quicksort is fast on average and well worth using.
As for other things that hackerrank doesn't test, I would say you should use hackerrank to screen applicants vs as an end-all-be-all.
The problem will be poorly defined, missing constraints, have useless test cases. Instead the 'secret' to answering many questions requires buying test cases, etc.
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Don't get me wrong, it's fun for some occasional practice.
I just don't see much utility in terms of solving 'real world' problems. Unless a company has a specific need for micro-optimizing their data processing.
In practical terms, it's usually better to use 3rd party tools that are battle tested, stable, well-tested, and highly optimized. The best code, is the code you don't have to write yourself.
Developers who would rather roll their own solution and rely on naive, yet, well known solutions to common CS problems may do more harm than good. Especially, in a time/resource constrained environment.
Some of the complaints are that HackerRank doesn't test things like team work. Of course, it doesn't! It's a good tool to screen developers. Things like team work, creativity and domain knowledge can only be tested in interviews.
I am not saying HackerRank has no issues but, discounting the importance of algorithmic problem-solving is very unwise, imo.