There is no doubt that Tidal energy is robust because the tide comes in and out each and every day. However, tidal energy on a large scale is not yet economically viable. In order to produce large scale energy from tide, huge infrastructure is required, including blighting coastlines. No such investment is under way, signifying it is not viable.
The tide is powerful and repetitive so there is no reason to question how robust that part of it is, but there is reason to question how robust the manner we use to harness that energy. The economic feasibility is also a major concern especially when taking into account the potential impact on natural as well as human interests and conflicts. We need to continue to investigate this resource as well as all alternative options.