Someone skipped preTS lol.
But well, it's clear what is this about given the example you used to complain about...
Sanji barely got time against Doffy since he got caught in Parasite quickly, so it's not like we know for sure if he couldn't hurt him with other techniques. So starting from there, you are already wrong.
And that was some arcs ago. They get stronger, for no reason, in each arc (in each arc we got better feats from them).
Haki is not something impressive or relevant unless you bring it on a high lvl or one learns the advanced techniques. For most of the other people who can use it, it's a small boost. It can be 1,2,5,10,20 % increase in AP for example, and durability.
It's not something much more impressive (unless like I said one upgrade it to higher lvl) than Franky's weapons, Jinbei's Fishman techniques or Robin's clutch techniques (she actually should be able to submit much stronger people, you know, that stuff about a person being able to lift Earth with a long enough lever, Archimedes, well, Robin's techniques works with those laws).