This assumes all fusions are alike, though. Trunks and Goten went from base SSJ , all the way to SSJ3. With more training, they might have taken it further and eclipsed Super Buu entirely.
There's no real proof Buu could analyze Goten and Trunks pre fusion, at base levels, and tell "hmm, they'll create a SSJ3 level fusion. I may be in trouble here.."
Goku and Vegeta's potara form was obviously far beyond SSJ3 Gotenks at base SSJ (And possibly at base, even if that was filler). Kibitoshin may not be an expert in Potara, but he saw enough of their powered up states to judge their fused form was well beyond what it should be.
And there's no real evidence that the potara has anything to do with it. Granted, there's no evidence either way, but that's why I say there's room for dissgreement..
Imo, it's a topic as nebulous as, say, whether the Thundercats are stronger then the Gargoyals, due to lack of solidly defined limits (Showing my age with this example. 🙂 )