To be fair, BB and his goons were caught off guard by Magellan. They were just smack talking, and Magellan just attacked them out of the bat. These characters are, in canon, goofy and careless fighters that get caught off guard unless truly serious/locked-in.
Magellan's canonical power level is ~ DF-less Shiryuu.
With Ivankov, there was no such ambiguity. Magellan just straight up defeated on guard Ivankov in combat easily.
I can understand the argument that Dragon's RHM and LHM are to be monsters like those of Roger or Rocks.
But they have to prove it first, before we as readers can consider accepting it.
Because, it would be one thing if they were simply unknown factors without feats and portrayal, we could then speculate them to be top tiers even.
But here, we have enough data.
Kuma and Doflamingo were introduced together, with Doflamingo having slightly higher bounty. Portrayal there IMO was, they are close in power, but Doffy a bit stronger. At no point in the story did Kuma give even remotely comparable energy/aura as Rayleigh (Roger RHM) or Whitebeard (Rocks RHM).
Ivankov's aura again doesn't even come close to Gaban. He wasn't even considered dangerous enough for level 6, they put him in level 5.....I am willing to overlook it. Then there is him getting taken out by Magellan, and in Marineford he didn't get anyone gassing him up either.
There is more of an argument for Lucky Roo (potentially mid gen Gaban/Sanji) being super strong, than Ivankov lol.
The most we reasonably can scale Kuma and Ivankov to, is varying commander level in power. I can buy Kuma being upper commander level, that's really about it.
Sabo is different for sure though. He will get to top tier, and is Luffy's "brother".