I’d never heard of Replay before, but I just looked it up, and I see it also uses RVC voice models to replace a voice. The big question mark, though, is how to find licensed RVC models, in the sense that the person whose voice is modeled has agreed to the use of their voice (and, at last potentially, has been compensated as part of that agreement – recognizing, of course, that some people might be okay with essentially putting their voice in the public domain, or some other arrangement – Creative Commons? – that doesn’t involve payment for the use of their voice)?
One of my collaborators is really into RVC voice models, and he familiarized me with some of this (even using a model of his voice to “sing” one of my songs as an experiment). But, when I looked into a few sites that were repositories of models, there were all kinds of voice models that were clearly not authorized (e.g. popular singers, cartoon characters, politicians, etc.). And, just now, in looking for information on RePlay, a short video I saw was using a Tom Waits voice model – I’m guessing that was not authorized. 
That is the consideration that made ReSing appealing to me for this application as IK deals with the business side of licensing and compensating the singers whose voices are modeled. IK was also smart enough to create a free version, and use of the free version (which only includes two voices, one male and one female, and a couple of instruments, and I believe restricts import of RVC voice models to one), which let me check out how well it would work on one of my recordings and thus get me interested in a higher-level version if/when I could afford it. (I ended up picking up ReSing MAX back in mid-May, when IK had a 60% off promotion on all their software, for $80!)
In the past, I’d tried a whole bunch of other techniques to change my voice to get a similar effect for background vocal layering, including iZotope Nectar’s Backer module, but the results decidedly left a lot to be desired (and sometimes were pretty cringeworthy). I’ve had much better results with ReSing thus far (at least once I find a voice that works for whatever context I’m for a specific part in a specific song), and the tuning options ReSing provides can also make a difference on that front.