I haven't invested with Roofstock, but I've done some research into a couple of deals I've been interested in on there. It seems like they're charging quite a bit higher prices than what nearby houses are going for that were listed through MLS, e.g. a house listed in Columbia, SC (needing ~$5k worth of repairs) being listed for $60k when its neighbors had been selling around $30k (repair needs unknown but Zillow showed similar condition). It seems like almost none of the listings would cash flow if you purchased with a standard 20%-25% down investment mortgage. Having not used the service, however, I have no idea to what extent you're able to negotiate down their list price.
I too am super intrigued by the concept, but not quite sold on the execution. I'm very much still in the research stage and gearing up to hopefully make my first long-distance rental purchase in the next 1-2 years (I'm in a HCOL area). There are so many variables to understand; it's a bit daunting.