Why do certain packages fail in NetInstall?
There can be several reasons for this. Probably the most common is that installer writers may not consider the possibility that their package might be installed on something other than the startup volume (i.e. with NetInstall, you're booted from a network volume but installing onto the local HD). The Installer application knows how to put the package's install files where they need to go, but if there are any associated scripts and such in the package, they may not cope with running from one volume and installing onto another.

Another (closely related) problem is that the package may have scripts that depend on resources that don't exist in a NetInstall environment. NetInstall runs in a pretty stripped-down version of OS X -- it includes everything most package scripts would need, but if they need something unexpected (like Finder, for example), they will fail and possibly cause the install to fail.