The first stage through HackerRank was fairly trivial. The one to one phone interview not so much.
They are a fast moving company and the interview process reflects this; you will need a speedy, knowledgable mindset to fit in there. If you are unable to present a trivial solution they are seeking in about one minute, and end up being hand walked through the solution, they probably aren't going to hire you, which is what happened with me. This is disappointing, as in retrospect, I know my stuff, but just didn't really apply it accurately (and quickly) to their questions. Or, possibly, I didn't apply it in the way they expected and wanted.
However, I specifically wanted to code solutions in Python, but yet in one question, it was hinted that this would be easier in the shell. But that's not necessarily where I wanted to be in terms of my employment (this is actually where I want to be moving away from). So using shell scripts, with grep and regex, to parse millions of lines of CSV files, to generate histograms, is not necessarily going to be interesting work (a presumptuous statement, no doubt, but this is the opinion I've taken away from the interview).
After reading their feedback regarding my interview, I found it somewhat constructive but peppered with presumptuousness. They expected more from me, considering my background, but at the same time, I expected more from them. As another review has said, the further away your skill set is from their very narrow specific set of requirements, the more friction you'll probably encounter during the interview process, and having reflected on some of the questions presented, I feel the role may be more about data processing rather than infrastructure engineering.
My overall opinion of Smarkets is that they avoid wasting time during the recruitment process, both theirs and yours, which is good (thank you!). But I also feel that they believe they are bigger, better and smarter than what they really are. A comparison was made by the interviewer that Smarkets > Facebook. Make what you will of that.