It was said that Kefla was strong enough to kill Ultra Instinct Goku when they fought. Also, a lot of power scalers in the Dragon Ball Community are saying Kefla is even stronger than Super Saiyan Blue Vegito from the Future Trunks arc. How is that possible?
Well for starters, let us examine the factors that influence the strength of a Potara fusion.
- The first obvious factor would be the characters who fuse/ take part in the fusion. It would logically make sense for a fusion between stronger characters to be > than a fusion between weaker characters. For example, a fusion between Goten and Trunks was Effortlessly pummeled by a Base Vegeta on account of his superior strength.
- A potara fusion also stacks additional multipliers. In the Buu saga when the fusion was first introduced, Goku contemplates fusing with Hercule who is substantially weaker than him. This proves that there is still some benefit from the fusion. More information regarding character Gokule can be seen here
- Finally, in Episode 114 in DBS, Shin states that a fusion between Android 17 and 18 would be ideal as they are Compatible. So one can assume maybe the compatibility between the charaters result in a superior fusion
With regard to whether Kefla is stronger than Vegito
- The Vegito from the Buu Saga is obviously weaker than Kefla. Caulifla and Kale are substantially more powerful than the Goku and Vegeta from the Buu Saga. Hence, it would logically make sense for Kefla to be stronger.
- Based on the above logic, it would obviously make sense for Vegito from the Goku Black arc to be several time stronger than Kefla. SSJ2 Caulifla at the very least is comparable to SSJ2 Goku/Vegeta. Kale on the other hand at the very least, is comparable to SSJG Goku. Even if you consider Kale to be equivalent to SSJB Goku or Vegeta/ it still wouldn't make a difference as Caulifla is clearly no where close to Super Saiyan Blue.
- The next main factor is based on compatibility. The compatibility between Goku + Vegeta and Kale + Caulifla is more or less identical as in both cases, they belong to the same race and also belong to the same gender and there really isn't any reason to indicate Kefla is a much more compatible fusion in comparison to Vegito
Also, I don't believe Kefla was ever strong enough to "Kill" UI Goku. This misconception stems from comments made by Krillin and Roshi. I believe these comments were made simply based off concern and that Roshi and Krillin clearly have no idea with regard to Goku's level of power. During Goku's second fight with Jiren, the latter uses a punch which Goku describes as the strongest attack he's ever seen and the power of which scares goku out of his SSJB form. Goku then proceeds to tank several of these punches one after the other in his base form. For future reference, I think it would be much safer to take Whis's statements at face value and possibly even the statements made by some of the Gods of Destruction as the level of power being utilized by Goku is far beyond something which Roshi & Krillin can comprehend.
IMO, Kefla is stronger than Vegito from the Future Trunks Saga.
Kefla in super saiyan form was in a similar level to Goku Super Saiyan Blue Kaioken x20. They exchanged hits, once Kefla was sent flying to a rock, hardly hit, and in another hit catching Goku kinda off guard, she knocked him down in Super Saiyan Blue Kaioken x20. Super Saiyan Blue Kaioken x20 is the power of 20 times the power of a super saiyan blue. So when she turned super saiyan 2, she had around the power of 40 times a tired super saiyan blue Goku.
Vegito from the Trunks Saga had the power of 2 super saiyan blue multiplied "tens of times". It could be 40 times, 60 times, 80 times etc. But since then Goku and Vegeta received several senkais. Goku since then had the zenkai from recovering from Zamazu, the zenkai from fighting with Piccolo and Gohan, the Zenkai from recovering from the battle with Gohan, the zenkai from the fight with Freezer after bringing him from hell, probably another zenkai from the energy Freezer gave him after fighting Jiren. And IIRC it was said by Jiren that Goku and Vegeta were stronger than the ones he fought before. So the fusion of a super saiyan 2 (Caulifla) with a legendary super saiyan type saiyan who walked through a super saiyan blue kamehameha (Kale), who was called a monster by a super saiyan blue Vegeta, and who catched the attention of Jiren to knock her down, has to be incredible powerful. Other people will disagree, but the berserker version of Kale is stronger than a super saiyan blue imo. Then the fusion of her with another saiyan can be stronger than the fusion of 2 super saiyan blue.