Please see the devel list thread for more info. There are a few issues that need to be ironed out:

• if the requested memory is too high, the macro may result in 0 threads effectively breaking: this should not occur. It should at least give 1 thread.
• the way the macro is written also seems to cause issues with using %{?_smp_build_ncpus} in %if conditionals: the spec file does not parse at all
• it is also undocumented that this must be used in the %build section of the spec, otherwise the spec won't parse either.

Giving negative karma to prevent it from going stable automatically.