Firstly, as PPs pointed out, unless someone has told you they identity as cis it is not ok to assume they are. At best it's presumptuous and offensive, at worst it's negating and distressing for people who may appear outwardly to be cis but in fact are not.
Secondly, "cis" hides that privilege and power dynamic comes from sex as well as gender.
Bluntly, a cis man is male and a cis woman is female.
In general, taking a male and female from the same social group (class, religion, health, race etc), the former has significantly more physical and social power than the latter.
A trans woman, a person assigned male at birth, spent at least part of her life - and in the case of those who transition later in life, a significant portion of her life - benefitting from male privilege which a cis woman has never had access to. For trans and cis men the underlying sex privilege is exactly reversed.
Using cis to describe both groups as if the degree of cis privilege is identical and universal obscures the different levels of underlying sex privilege cis men and woman have relative to their trans counterparts and is therefore unintentionally sexist and disadvantaging for female people of all genders.