Cochran, Orr and Teichner introduced $L^2$-eta-invariants to detect highly non-trivial examples of non slice knots. Using a recent theorem by Lück and Schick we show that their metabelian $L^2$-eta-invariants can be viewed as the limit of finite dimensional unitary representations. We recall a ribbon obstruction theorem proved by the author using finite dimensional unitary eta-invariants. We show that if for a knot $K$ this ribbon obstruction vanishes then the metabelian $L^2$-eta-invariant vanishes too. The converse has been shown by the author not to be true.