Betrayers and Adventurer is not a very good combination at higher difficulty levels. It depends too much on chance. Sure, if you see Thannata, and manage to research the tech to hire her before someone else kills/hires her, you get a huge boost. But on ridiculous, the AI expands very quickly, and knows where the heroes are, and beelines for them.
In my last playthrough, I had "Betrayers" and "Wealthy", not because I was going for the combo, but because people challenged me to win with standard Karavox on ridiculous. How much did "Betrayers" help? A tiny bit.
First of all, I never saw a high level hero that I could hire. I hired my starting level 1 hero, a level 3 hero that was close to my starting location, and a level 3 who was in an impossible to reach without fighting area. There were also three level 5 heroes who ended up deep within my borders, so that I could keep them out of AI hands. At some point, while I was engaged in a five front war, I hired two of them. I did not bother with the third, despite having plenty of cash.
On high difficulty levels, heroes just do not do so much, unless they are really high level. My 'main' heroes ended the game at levels 23, 21, and 19. The secondary heroes led troops and barely reached levels 9-10. Sure they were useful, but not very influential.
In .992, ridiculous has been made more ridiculous. I would say to the point of being a chore to play. Betrayer+Adventurer would be a inefficient combo. Heroes need to be power leveled to matter when your opponent is rushing everything in one turn and can put 8-9 traits on units, doubling traits as it feels the need. You cannot powerlevel too many heroes, and the hero techs are a waste of research otherwise.
So... Effective at lower difficulties? Yes. Overpowered at high difficulties. No.