I have seldom been asked by an interviewer to provide an expert review of the client's product/service as part of the interview process.
I would assume that this is a good opportunity to see how a UX designer approaches the potential task of improving the user experience, and also a good opportunity for the UX designer to see the company's attitude and ideas about UX design.
And if someone working in a company truly believes that their product is the best and cannot accept any criticism or comments about the product, then there's no real point of actually hiring a UX person to improve the experience.
On the rare occasions that I have provided constructive feedback about the company's product/service during an interview, my impression is that these comments are seem as the UX person wanting to look important or appearing arrogant, when it fact the interviewer also contributes to this by not having enough understanding about UX.
Too often I see job descriptions posted requiring UX designers to do user research when it is clearly out of scope for the project, or for UX designers to have knowledge of Agile processes when the company does not implement any. I think it is really strange to accept or reject applicants based on what is written on a resume, because I often have problems believing what is written for a company job description.
What is the main problem with just asking the applicant to review the company's product/service, at least from the perspective of a potential user?