Translation of "featureless" into Spanish

They're so featureless, even they can't tell which one is which.

Son tan monótonos, ni siquiera ellos pueden decir quién es quién.

The caves themselves are dark, featureless, and without interest, except for a strange echo.

Las propias cuevas son oscuras, sin rasgos distintivos ni interés alguno, salvo un extraño eco.