The sign of a good cheesecake base is firmness but not rock solid; you want the knife to easily cut through the base.
Adding too much butter will cause a rock-hard base, so you want the biscuit crumb consistency to be like a ‘just-wet’ breadcrumb, if you can pinch a section of crumb together with your fingertips and it forms solid piece without applying too much pressure the base will be hard once refrigerated.
With baked cheesecakes, unless your mixture is very moist, don’t over bake the pre-baked base – this will cause it to go hard, when it is baked again with the cheesecake filling.
Press the base is another important step. Use the back of a metal spoon (wet with a little water) to lightly press the crumbs into the base. You can use the bottom of a glass for this also.
When building a biscuit base that goes up the sides, start with the bottom base first. Next add heaped spoonful’s of crumb mixture around the edges of the tart tin, then press the mixture up the side of the tin (I find this best using the edge of a round glass).