Question

Why are using some special characters (@, #, !, etc.) not a good idea?

Answer 1

In Windows you cannot use #,!,@ (There are more, I just don't remember them all) in a name of a folder. 

In some websites you cannot use special characters. Some characters are what you "escape" characters.

An escape character can cause an error when trying to insert data or get data from a database. Sometimes it can prevent a syntax error, if used the right way.

Answer 2

There are pros and cons to using special characters in email subject lines. Generally, marketers report higher open rates.

Some report better engagement, but many don’t.

There are also reports of special characters causing problems with deliverability, mostly because spammers became very fond of special characters for a while.