I think a better way to phrase it is that judging an engine by the games they've been made with it will give you an incomplete picture: you can only see what is

I think a better way to phrase it is that judging an engine by the games they've been made with it will give you an incomplete picture: you can only see what is possible, not what is impossible (and even with the possible, you only see what the people use it can do which is affected by their resources and their skills). It is better to evaluate the engine yourself for the project you are planning to do with the requirements it has (not making the entire project, of course, but if you - say - want to create a huge detailed world you can procedurally place entities in the world and see how the engine copes with that and if it provides tools to address any issues).