Similar to @Moistbushes, I could see my ex needed therapy for issues stemming from a bad childhood for a long time before he finally went at my request. He's been going for years (I think he still goes) and he's a little better than he was, but I think he's too invested in the image he has of himself to be properly honest with the therapist and confront himself. I know he hid a major issue from his therapist for months while we were still together and the only reason he started talking to her about it was that I discovered a whole lot of evidence of his behaviour and he was forced to confront it. He has other issues that I am sure he will never admit to/bring up with his therapist as he is stuck in denial/blame mode.
If someone is not there for the right reasons (ie is really just going to "prove" to themselves that they're a victim and misunderstood/under-appreciated and everyone else causes their problems) then they won't get much out of it. Maybe some surface-level tools. Worse, if they are very dishonest with themselves, they will twist and weaponise things their therapist says to use against their partner/other people.
During our breakup my ex claimed his therapist said I was controlling. I know I am not, and I also know a therapist wouldn't be likely to make such a definitive statement about someone they're not treating. I - sincerely - said I didn't think I had control issues but if his therapist was that sure about it, then perhaps I should make my own solo appointment with her so I could understand things through the eyes of a third party. I asked him what exactly she had said, so I could discuss it and all of a sudden "She says you're controlling" became "Well, she said it sounded as though you might tend towards over-control".
My ex would often accuse me of being controlling, but could generally not give examples of how I was doing it. Sometimes he would twist/exaggerate/make up something I'd said or done and claim it was controlling and refuse to accept anything to the contrary. (Of course, in one-to-one therapy, the therapist will only hear the twisted/exaggerated/made-up version.) I could tell from what his therapist really said that she was trying to validate his perspective of 'feeling controlled' without taking what he was telling her as gospel. But he twisted her actual words to serve his own narrative, just like he used to do to mine.
That's the trouble with people who have deep-seated issues - they deploy the same toxic strategies (lying, misrepresenting, blaming, stonewalling, self-aggrandising etc) on their therapist as they use on everyone else and it makes genuine progress very difficult.