My impression is that some abilities of a df can be easier blocked by haki than others.
Hold that thought there.
For example, it seems that Law's "shambles" only works on targets with no or low haki (Tashigi), or on people who don't see it coming (Sanji & the others).
It would explain why Law didn't immediatly "shamble" Smoker at first sight - which would have saved him the trouble of actually engaging in real combat. This tells me that he had to catch Smoker off-guard in order to use the body swap, with a strong "single target" attack: MES.
I think that "strong" attack is actually Law concentrating his power into a small area–his scalpel. It's similar to how Luffy was able to activate Gear 2nd locally in his arm and arm only his hands with Haki. BTW, due to the strength of these "localized" powers, I think Luffy focused power to small areas on his body, meaning that strength that would normally be distributed throughout his body can be concentrated.
Also relevant, the Admirals used Haki to protect a small structure from Whitebeard's attack. It seems Haki can also be projected over wide areas as well, although doing so may leave the Haki user immobile as long as their "shield" is up.
After Smoker passed out he could basically freely use shambles without any resistance.
This theory would also explain why Hancock uses 2 abilities which do basically the same thing:
1. The mero mero beam
2. Slave arrow
The mero mero beam could function as the widespread attack, which basically can't miss due to its range. Downside would be that it is only usable on fodder and targets who don't posses haki.
The slave arrow's have quite the range as well, tho they actually have to hit the target. However, they could have an impact on haki users, maybe even piercing the invisible armor. Thats what arrows are supposed to do, right?
For the hardcore haki users both attacks might not be suffcient enough and she needs to engage in direct combat to petrify, namely with parfume femur. On absolute masters (Rayleigh/Shanks) the whole DF might cease to work.I think this makes a lot of sense.
I am not sure if I understand exactly what you mean by "absolute" masters like Rayleigh and Shanks. The way I see it, Rayleigh and Shanks are so good with Haki that no DF power seems to work on them, but that doesn't mean that DF powers won't work at all on them. If Hancock's Haki proficiency were anywhere near their's, she would be able to use her powers on them. (That would also make her Emperor level and even one of the strongest of them.)
Or perhaps, it's a combination of CoO and CoA localization. If Hancock shot a slave arrow at a Haki user who focused CoA on their arm and then blocked the arrow with the arm, it would become a matter of focused DF strength vs focused Haki strength. I have a feeling that Rayleigh and Shanks wouldn't even have to focus their Haki to resist any of Hancock's powers.
I also wonder if having and training a DF power means a trade-off for Haki power, simply because you have to spend time with each power. I think Luffy can train both at the same time because his only power is having a rubber body, so he can train in Haki like a normal person. However, a Logia user would have to balance Haki training and DF training, because their powers don't just limit to their body. Paramecias that produce a substance or an effect would have the same problem too if their DF allows expansion of their powers.
Actually, now that I think about it, Luffy can train his stretching power, and that would take away his Haki training. I think training physical strength would also take away time from Haki training. Or training in special martial arts. This decision-making in training will result in a wide variety of builds, in a gaming sense.