As a guy, I find it incredibly stimulating to be tasting pussy while receiving a blowjob, and have cum many times while doing it. So that disproves any “impossible” urban legend, which sounds like something an inexperienced teenager made up. However, “difficult” may be accurate for most women. The women I’ve 69ed with usually seem to find that having a cock in their mouth distracts them from getting to the happy place they need to be to cum; they usually release cock out of their mouth, panting, as they start getting really worked up.
Think about what it takes for women to reach orgasm during coitus. My (limited, but far from zero) experience with different women is that some reach orgasm much more easily while on the bottom and able to fully relax, whereas others have a strong preference to be on top and in control. So how do those pathways to orgasm play out during 69? Both are blocked. Even if she’s on top, in a 69 a woman isn’t really in control of her stimulation when she’s receiving oral, is she? Your mouth is. Conversely, is a woman who really needs to relax to cum going to be able to that on the bottom of a 69? With a guy hovering over her and thrusting his dick in her mouth? Unlikely.
I think for most opposite-sex couples, 69 works better as a form of foreplay than as an ending. And I say this as someone who absolutely loves 69 and recently (as my 69th story) published an ode to the act, and has always found it rather depressing that most women don’t seem to like 69 very much.
On the other hand, as Dominica shared, 69 may be a lot better for same-sex couples, the mutuality of being able to do the same exact thing to each other at the same time, or to do on the other person precisely what you want done to you in that instant, leading to potentially better, and better-synchronized, orgasms.