? While some people say that it is a good thing, others deem otherwise.
Strictly speaking, it's grammatical. However, it's unidiomatic and unnatural, and it wouldn't normally be expressed in that exact manner.
From Merriam-Webster's definition of deem:
transitive verb
: to come to think or judge : CONSIDER
// deemed it wise to go slow
// those whom she deemed worthy
// a movie deemed appropriate for all ages
intransitive verb
: to have an opinion : BELIEVE
Following that, if you substitute the intransitive and synonymous believe, you end up with this:
While some people say that it is a good thing, others deem believe otherwise.
However, the intransitive use of deem is rarely used, and I've certainly never seen it used like this.
It would be far more natural, and quite common, to use the transitive version of deem:
✔ While some people say that it is a good thing, others deem it otherwise.
This is not only much more idiomatic syntax, but it also preserves stylistic parallelism with the use of it in both parts of the sentence.