Janemba is weaker than Super Buu
This is just my speculation but I dont think Janemba was as strong as Super Buu /w absorbtions. In the movie Goku when Goku transforms into ssj3 he says something like "Majin Buu was the only person to push me to this form" making me think that this movie occurs in an alternate timeline. Goku mustve defeated Fat Buu (he only expressed doubt about beating fat buu) and went back to the afterlife. The fact that Goku and Vegeta are both dead further supports my thoughts.
im pretty sure it said somewhere that potaras fusion>normal fusion.
not to mention it stays for eternity, while normal fusion is like 30 mins.
so my money is on vegito (if thats the potara guy)
also he looks cooler and is the only canon one U_U gogeta never came to be in canon universe i think
I was gonna say because SSJ4 too. But he vaporized Janemba who was stronger than Buu in one attack.
Okay, I'll try to see if I can explain my retorts to all of this as one, combined post.
Well, first, we need to look at Fusion Reborn as though it's canon, while it may not be. Goku commented that "No one has ever pushed me this far except Majin Buu." This, coupled with the fact that Goten and Trunks know the Fusion Dance, which wouldn't be possible if SS3 Goku actually defeated Fat Buu, show that Fusion Reborn takes place in some alternate timeline where the Buu Saga happens, but Goku and Vegeta remain dead. This means that Goku has fought Kid Buu, who is about equal to Super Buu with no absorptions.
Goku commented at one point in the movie that he's never felt this much power in one place before, referring to Janemba. The fact that he's fought Buu and the fact that Janemba(in his red, slim form) completely stomped Goku who matched Kid Buu in combat power supports that Janemba is stronger than Kid Buu, and Super Buu.
It was never stated that the Potara Fusion is stronger than the Fusion Dance, but that it was a more ideal method of fusing for the situation, as there was no time limit and it there is no long process with the former method and can be done quickly.
SSJ Vegito completely outclassed the strongest form of Buu, and could have killed him quickly. The fight only happened because Vegito was toying with him so he can weaken his body and get absorbed so he could free his friends and family. If he had just killed Buu like Gogeta did to Janemba, he would have killed his friends and family too.