I'm one of the authors. It's interesting that you cite our paper as an application of the NIST beacon. Perhaps a better reason to cite our paper is that we actually design a beacon using the Bitcoin blockchain itself (Section 4.4), because the NIST beacon has many problems.
The other major one is reliability (but that is encompassed by sufficiently broad interpretations of "trusting NIST.") Also, for our specific application it's not clear how to assign precise timestamps to Bitcoin blocks that everyone can agree on; this is necessary to be able to use a beacon that is external to the block chain.
