DJ got out of hand a bit post-ts, Ill give u that but pre-ts was pretty okay I mean he would use the friction to create fire and that's why he was shown spinning until his leg caught fire I mean its not like that would work in real life whatsoever but still its a decent enough explanation for a fantasy manga but the whole hells memories and other things sure got out of hand post-ts, but there is also the comedic relief fact when Sanji turns all SSJ when he gets angry around Nami etc but rarely gets full body fire when hes just normally fighting thats just bizarre if ODA goes that way.
Also again I doubt u can find any big flaws on pre-ts DJ because as I said him causing heat which results in fire is more than enough of an explanation for a shonen manga, like cmon its just as u said the manga is filled with crazy shit and this is pretty acceptable and of course people would risk the DF because mera mera no mi is first of all logia and the fact that u can use it anywhere in ur body and even shot fire at ur opponent but Sanji can use it only on his leg for a short time due to the heat caused in his leg but that was until post-ts where it went all to shit.
Also again I doubt u can find any big flaws on pre-ts DJ because as I said him causing heat which results in fire is more than enough of an explanation for a shonen manga, like cmon its just as u said the manga is filled with crazy shit and this is pretty acceptable and of course people would risk the DF because mera mera no mi is first of all logia and the fact that u can use it anywhere in ur body and even shot fire at ur opponent but Sanji can use it only on his leg for a short time due to the heat caused in his leg but that was until post-ts where it went all to shit.