Unfortunately, It was one of the worst interview experiences during my whole career. I was silly enough not to stop my interview process after the first interview full of the red flags and got persuaded by a recruiter to carry on.
The recruiter has been mentioning "Mettle culture" all the way, but don't buy it - there is no culture. You don't have time to ask any questions during interviews, it's only about Mettle engineers to bombard you with all sorts of questions for 3,5 hours. I personally believe they were not given any proper training on how to interview people, hence all these failures and negative reviews.
They do look at how to fail you during the interview, it's not a 2 way conversation between engineers at all.
When I was doing a coding exercise I got this in the Readme file: "The objective is to ensure there is enough test coverage, fix any flaky tests and fix any bugs you might come across."
When they rejected me their feedback was that I focused on the tests only and would struggle with other aspects of a SDET role.
So they ask you to fix tests, and reject you when you do so. They don't check how you would deal with other aspects, but just assume you wouldn't be able too.