Unitarianism was a very mainstream Protestant denomination in the nineteenth century. It later left the orbit of chr*stianity altogether.
Christians all acknowledge one God in three Persons, the holy Trinity (Father, Son and Holy Spirit)...Unitarians by definition (hence the name...) do not.
There are also "Oneness Pentecostals."
There are two kinds of "unitarians": fundamentalist unitarians (like the OP's) and liberal unitarians (like the UU's). While both reject the "trinity," they are still as different from each other as can be. One rejects the trinity because it believes J*sus was a mere human (and thus rejects chr*stian soteriology, the miracles, etc.) and one simply believes J*sus alone is "god" and that Heaven was vacant while he was on earth. This group accepts soteriology and all the miracles.
All well and good. None-the-less even if socially, Unitarians were seen as mainstream in the 19th Century (especially in New England where they started), and some were more radical than others.... no orthodox theologian has ever regarded any group which rejects the holy Trinity as an actual Christian group. Yes, that does apply to Oneness Pentacostals, who, while not unitarian.... repeat the (very early) heresy of modalism.
Can an individual be saved who denies some essential of Christian orthodoxy? God knows. All I can say is that the early creeds were designed for a reason: To divide Christian (groups) from non.