Buying or exchanging links for ranking purposes can go against Google's spam policies on link schemes. When you resell link work, that exposure sits with your client's site and, in terms of reputation, with your agency. Careful, relevant, editorial-style placements reduce the risk but do not remove it, and we say so in every scope.
Links are one signal among many. Results depend on the client's site, competition and category. Reports show what was done and where, not what rankings will do next. To see how we think about vetting any provider, read the local link building service guide.