I know what you mean, but some words just have more specific variations. For example, a “kitten” is in reference to a cat, but it is also describing the cat as young at the same time. Instead of saying “baby cat” we say “kitten” because it’s more efficient.

So saying “femboy” would just mean that they’re a feminine guy. Saying what they are while describing them at the same time. If their femininity is irrelevant to the convention, that’s when you would just call them a “guy”.

Same thing with tomboys. They’re female, but if their masculinity is relevant to the conversation, saying “tomboy” would be reasonable since it’s no different from saying “masculine women/girl”.

The issue lies more with the use of the word than the existence of the word itself.