Despite its reputation, the Spirit Bomb has only worked once in the Dragon Ball universe. Out of all the times Goku has used it, the only time it accomplished its purpose was at the end of Dragon Ball Z when he launched it at Kid Buu. Kid Buu was the first – and so far the last – Dragon Ball villain defeated by Goku’s strongest attack.
Of course, that’s not to say that Goku hasn’t had plenty of opportunities to use it. As the most powerful technique in his arsenal, the Spirit Bomb has been deployed a total of four times in Dragon Ball canon, all of which being situations where the Z-Warriors had very few options left. Against Vegeta, the all-powerful Spirit Bomb was presented as Goku’s trump card, but it failed to finish off the villain. It didn’t work on Frieza or Jiren either. When he originally learned it from King Kai in Dragon Ball Z’s Saiyan Saga, there was a feeling that any opponent wouldn’t be able to walk away from a direct hit from a charged-up Spirit Bomb. And though it is true that the attack is superior to all of Goku’s other abilities, it just hasn’t served him as well as the Z-Warriors would have hoped.
The repeated failures of the Spirit Bomb don’t necessarily mean that the attack is overrated. Instead, its lack of success can be attributed to other factors at play. The Saiyan Saga's Spirit Bomb didn’t work because Vegeta landed an attack on him during the energy gathering process. As a result, the Spirit Bomb lost a significant portion of its energy before it was able to hit Vegeta. The second use of the Spirit Bomb went wrong on account of the destruction that Namek had already suffered. For this version of the attack, Goku was having to channel energy from a planet that barely had any life left on it. As for the situation with Jiren, Goku was only able to rely on the energy from his Universe 7 teammates (Vegeta excluded). This just wasn’t enough to take down Universe 11’s strongest fighter, who was much more powerful than even Goku at the time.
All things considered, the Spirit Bomb is an incredibly potent ability and arguably one of the strongest attacks in the Dragon Ball franchise as a whole. The main issue with it is that getting it to work requires certain conditions to be met. The first challenge is charging up the Spirit Bomb. That takes time, thus allowing Goku’s enemies a chance to interfere with the process, hence what happened with Vegeta. There’s also the matter of what energy Goku has available to him. That held him back against both Frieza and Jiren. To beat villains as powerful as them, he would have needed a lot more energy than what he was working with.
With Kid Buu, on the other hand, the circumstances were a lot more desirable. With Majin Buu and Vegeta buying him the time he needed and the entire population of Earth lending their energy to Goku, the third Spirit Bomb was more than enough to take out the villain. It remains to be seen if the fight with Kid Buu will be the last time everything lines up so perfectly for Goku and the Z-Warriors in the Dragon Ball anime.