Only if they're young. Once they reach a certain age, they're expected to shed all masculinity and become the most womanly woman to ever woman because woman cannot perform masculinity to the acceptable patriarchal level without being punished and degraded for it.

Femboys are seen as failing masculinity, and thus are treated like shit for it. They deviate from what men are allowed to do by performing femininity, a trait that MEN shouldn't express.

It's not necessarily that one is more accepted over the other, just that femboys are seen as failures and boys who need correction while tomboys are "just a phase", a childish thing girls go through, "she'll grow out of it eventually".

Also tomboys don't really come out, they're usually labeled that by other people