In a perfect world, which my novels are supposed to be, I guess, since I'm creating them - internal monologue is reserved for secrets my POV characters don't want to divulge. My strong preference is for short, to the point thoughts that subsequently show on said character's face. Ari, from my first book, Enemy Within, did a pretty good job with that - internal monologue that was focused and specific and usually about something she didn't want anyone else in the universe to know. Most of the time that worked out for her. She managed to hide all kinds of hurtful things from her family. But then the hero came along and could guess she was keeping secrets that he subsequently took great joy in prying out of her. Her family eventually got the full story, too, because secrets kept mean the character isn't learning her lessons.
In that story, anything Ari thought was worth saying or it worth hiding. If she thought something, it was my job to push for her to say it aloud. I needed a compelling reason to keep it shut up inside her head. I've tried to keep that criteria in mind for every subsequent story and character. Some of them cooperate more than others.
In the book I just turned in to my editor, the heroine has abandonment issues. As a result, she doesn't engage with other people much. It's too threatening. That puts her in her head a bunch. I'm hoping the thing that keeps that from being hopelessly self-indulgent is that she ends up with an internal eavesdropper - a passenger sharing headspace. It was an odd balance to attempt to strike - her wanting to keep secrets, but being stuck with no place to hide them. At that point, why shouldn't every thought that comes into her head come out her mouth? I guess when the editor reads the book, I'll know whether or not I came anywhere near getting it right.
I don't hate internal monologue in any way. I think it's necessary in moderation. For me, it should serve two purposes: 1. It should tell me something I don't already know about the character and 2. It should reveal a goal, a secret or a wound. But for the story to move forward, those internal bits of information can rarely stay internal. Unless you're writing tragedy.
Internal monologue that remains internal creates no stakes. There's nothing at risk. With nothing at risk, there is no impetuous to change.
So somehow, that internal has to be forced to the surface via word or deed. And then, there's some argument to say why not do away with the internal and focus on word and deed? Eh. If the internal monologue heightens tension, all's fair in the name of story conflict.