From NachiWiki

With pre-hung steel entry doors, the adjustable thresholds are aluminum and are prone to bouncing when stepped on.

Care must be taken to either be sure it bottoms out during installation or if shimming is required under the threshold the shim should be solid. Not those silly little cedar shims but a shim cut out of a 2x4 or 6 the full width of the threshhold.

There should also be some type of support on the exterior where the threshold hangs over as a lot of deflection will occur there. Support would commonly be a 3/4 trim board. Could also be brick with no gap.

The door should close tight all the way around. Pre hung doors were supposed to make door hanging quicker and easier but people still manage to mess up the install and in some cases, especially in really old homes, the "old fashioned way" (skillfully hand built) is better.

This article was originally based a post by Cheremie S. Kratzer on the NACHI message board.