They pollute the namespace, and cause strange action at a distance.
This module will make any call to a ->which method in any loaded module which goes via an AUTOLOAD fail.
This same functionality would be trivial to add to something like Class::Inspector, without having to muck with UNIVERSAL in the process.
This, and pretty much every single other UNIVERSAL::something module, should never be used in production. There's tentative but reluctant acceptance of UNIVERSAL::isa/can because they CHANGE the behaviour of existing functions, but even they do not ADD to the APIs.
This module, along with things like UNIVERSAL::clone, do not belong on CPAN.