Central builds fail because they run out of files descriptors. This probably triggers other package failures too for packages built at the same time.
Tobias, both MAKE_JOBS_NUMBER_LIMIT and MAKE_JOBS_UNSAFE are listed in the same section For build and install in Mk/bsd.port.mk.
Further, they influence MAKE_JOBS_NUMBER in a similar fashion. They also have different purposes. MAKE_JOBS_UNSAFE is ... To be used with known bad ports. which is not exactly the case here.
What determines that one is global and the other one isn't?