Threshold of Shadow is the most popular pump in Fellowship Block, which isn't saying much. Given their high strengths and Bill Ferny, Nazgul are not in dire need of additional strength in the first place. To make matters worse, none of the pumps are free (save Dark Whispers, which has its own requirements) until set 11, and the extra conditions for additional strength are all very stringent. Threshold of Shadow has a higher maximum boost than Frozen by Fear, but lower maximum boost than All Veils Removed. None of the extra conditions for these boosts are likely to trigger for generic Nazgul builds, while specific Nazgul builds (such as corruption) don't need pumps. However, given bidding and the potential for the Free Peopls player to add burdens, Threshold of Shadow has potential for its +4 boost, and that boost can be handy in situations where it does appear.