• Some servers are case sensitive and if you forget capitalization in a url, it won't work.
• Some browsers and software don't handle special characters well
• The underscore character is harder to see when it is underlined,
• Some search engines rely on a dash (hyphen) to separate words and an underscore makes it look like one big word.
• Typing a hyphen is a single key-stroke, whereas the underscore requires the Shift modifier key.

Headers - Metadata:
Don't use stop works in titles: e.g.
"I, a, about, an, are, as, at, be, by, com, de, en, for, from, how, in, is, it, la, of, on, or, that, the, this, to, was, what, when, where, who, will and with."