I heard dating your coworker is a bad idea. Is that true?

I have a huge crush on my coworkers and I’m working up a courage to ask him out tomorrow. A friend of mine told me it isn’t a good idea and just forget about it. My older sister said there’s no harm in trying but if we do date that’s a huge risk we’re putting on ourselves and work place romances are a horrible idea. She said it’s up to be but it isn’t really a great idea.
Now I’m having doubts. What should I do?