TL;DR: As far as I can tell, there weren't any major issues with the core idea of Relationships and Dating. The site simply didn't attract a large community at the time. If Relationships can gather a strong community base, there is a good chance it could succeed where Relationships and Dating failed.
A.Danzi's comment brings up a good point. To answer this question, it's important to figure out why Relationships & Dating failed. So let's look the statistics from the Relationship & Dating beta and analyze where the idea could be improved.
Disclaimer: I was not an active member of Stack Exchange when Relationships and Dating was in beta, and as such have no firsthand experience with the site. I am merely speculating based on publically available information.
Questions per day: While 5.8 out of 10 questions asked per day isn't bad, it isn't ideal either. As Raghu's answer suggests, Relationships covers a broader topic than Relationships and Dating, which could allow for a more generalized range of questions.
Answered question percentage: Over 90% of the site's questions were answered. No problems here!
Active users: Here we have the first real issue with Relationships and Dating. Early on, the site lacked a strong base of active users. While it's hard to tell this early on, Relationships does seem to have a good number of high reputation followers.
Answer ratio: Once again, no problems here. Let's move on to the final, arguably most critical metric...
Visits per day: This right here is the metric that killed Relationships and Dating. Put simply, the site did not have enough visits per day. This could be due to numerous issues. Maybe 12 days in beta wasn't long enough for the site to grow and foster it's own community. Maybe the site didn't have a good base of dedicated users to start with. Maybe Google's search algorithms brought users to alternative sites to answer their questions. At the end of the day, there's really no way to tell.
In conclusion, even if the idea is very similar to Relationships and Dating, I think enough time has passed to give the idea another chance. (AKA: See TL;DR)