Well, this is more a matter of logic and some common-sense than "facts". It doesn't necessarily mean it's more easily bowled over than facts.
A cliffhanger demands resolution and satisfaction. It teases. A flashback is often experienced as a distraction, and subordinate to the main story. It also should be resolved efficiently and get to the point, without too much meandering and stretching, so there isn't too much frustration about the main story being left hanging. If the flashback is right in the middle of a cliffhanger, the potential for frustration is increased.
Generally, a flashback being too long can be considered clunky anyway.
Putting a flashback within a cliffhanger is a particularly extreme way to place it, due to how it increases the cliffhanger's tension and potential frustration with the flashback. So a cliffhanger and a flashback should never be combined in their most extreme ways, with the flashback seemingly taking forever, or the whole thing becomes one of the worst possible combinations.
It can be done sensibly, if questions and points raised in the one are directly taken up in the other, ultimately leading to a dramatic release. An idea that Oda seemingly wants to follow, but not only that, he takes the opportunity to tell about everything under the sun and several full lifestories, with everyday goings-on (e.g. so that we see Rocks asking Harald to join basically every chapter), with excitements far between. Within the flashback, he creates further teases, that he puts off until later, with deliberate slowdowns. The pacing is just not good.
Part of the reason is that Oda is obsessed with teasing the readers and putting things off indefinitely. He thinks it's super-aweseome. At some point it just starts sucking - or there is no need to pretend happiness. One can ignore it all with some indifference, or getting totally hung up on the occasional "hype" panels, as mentioned.