Two of the most fundamental distributed symmetry-breaking problems are that of finding a maximal independent set (MIS) and a maximal matching (MM) in a graph. It is a major open question whether these problems can be solved in constant rounds of the all-to-all communication model of \textsf{Congested\ Clique}, with $O(\log\log Δ)$ being the best upper bound known (where $Δ$ is the maximum degree). We explore in this paper the boundary of the feasible, asking for \emph{which graphs} we can solve the problems in constant rounds. We find that for several graph parameters, ranging from sparse to highly dense graphs, the problems do have a constant-round solution. In particular, we give algorithms that run in constant rounds when: (1) the average degree is at most $d(G) \le 2^{O(\sqrt{\log n})}$, (2) the neighborhood independence number is at most $β(G) \le 2^{O(\sqrt{\log n})}$, or (3) the independence number is at most $α(G) \le |V(G)|/d(G)^μ$, for any constant $μ> 0$. Further, we establish that these are tight bounds for the known methods, for all three parameters, suggesting that new ideas are needed for further progress.