it's painful, it takes a while to die, si usually gets to you and you/someone you know calls emergency services, it takes a ton of willpower... it's not a good method. it's portrayed in tv and movies so frequently because the media thinks that since self-harm via cutting is a thing, people kill themselves that way too.
stabbing is also a very unpopular method because of survival instinct. if you don't stab yourself in a way that kills you/knocks you out instantly (or within a few minutes), you're going to freak out and call an ambulance as you wait to die (unless you're the most apathetic person on earth).
in addition, it's very hard to stab yourself in the heart. your ribcage protects many vital organs and most of the time it does a pretty good job. you'll have to do a lot of research and know exactly where to point the knife. even then, when you're about to kill yourself you might forget all the information necessary to succeed and end up screwing up out due to nerves or, of course, your innate survival instinct.
not saying you should hurt yourself in any way but if you really do want to die then i'd recommend checking out the sn thread. from what i've seen it's a relatively quick way to end your life and the process of dying after ingesting sn only takes around 45 mins to an hour (though you're not conscious for most of it).