It doesn't alter the characters per line indication, so it doesn't mess with debug or syntax error messages if that's what you mean. The difference is only in the presentation, but "under the hood" it's still two characters. In Atom, with Fira:

* typing => turns into the ⇒ glyph automatically

* it still counts as two: a new document with only ⇒ in it and your cursor at the end reads properly as "Line 1, Column 3"

* visibly, if you're using a monospaced version of the font, it looks like a single character. otherwise, the "optical" width is completely arbitrary just like a regular font