First off, I think the bane of most of the BT type armies would be a D-field. If you are playing against Templar-like armies I would personally advise you to take a D-field on your Great Harlequin at least... and if you have the points.. on all your Harleys that are able to take them (all but basic troupes). I understand this is expensive, but it's worth it....
I also read somewhere, though I can't find it off the top of my head... that the D-field is tested every time the 'fixed roll' is attempted... which means that every combat phase that the BT player would choose to use the Vow, they would need to roll first to see if it works.
Imagine this combo. Max number of DJs, all with D-fields, Phase fields, and Shrieker cannons. Combine this with a mix between Dread and Rictus masks and make em all join a single unit. Lots of fun, though they do have a high price... but what would actually be a deadly force to deal with. For the hell of it, swap one or two of the shrieker cannons with Brightlances or Firepikes, maybe one or two with Hawks Talons. Tank hunting, hard to kill in HtH and shooting... etc..etc. Of course not forgetting the option to either move D6"+6" (normal move + Fleet of foot) or up to 3D6" ...