According to legal issues we deal with them the same as trinis - they are both considered "people of the book" in that sense since they both claim to follow the same or in actuality - similar books - since there is no universally recognized biblical canon by those who call themselves christians.
However, regarding religious issues we deal with the trinis as pagans because although they claim to be people of the book yet find nothing in that book to support their belief. So in religious affairs we realize they are people of their desires, people of customs & traditions, and various degrees of pagan rites.
The unitarians are part of not only the Abrahamic fellowship, but the Messianic one as well. Arguably even 'modalists' are unitarians. These people should be seen as modern-day hanifs.
Personally I would not eat meat killed by a trini or atheist. I eat kosher and halal meat - but I'd eat meat killed by a unitarian christian. I've also been invited to Unitarian church before to give a powerpoint presentation on Unitarianism from an Islamic perspective. They're very liberal though in the US, but the UK has a conservative unitarian tradition. I wouldn't walk into a trini temple or shrine.
I do believe at least 5 US presidents were Unitarians. Trinis have claimed ownership of a country founded by stoic deists and unitarians as they crash it into the ground with their nonsense beliefs that they try to force on everyone and people are sick of it, alhamdulillah.