Okay. Inspired by some (earlier, incidental) comments in here I decided to do some testing in the Keep myself, and I recognised two likely culprits to the DoT build being so effective (and yes, very likely too effective, though that I can’t give a definitive opinion until significant field testing).
First, Famished Flames is insane. Yes, it indeed gives 150% extra DoT damage ( I was suspicious of that, and that was what I set out to test because the numbers sounded unbelievable), i. e. all DoT does 2.5 times as much damage as usual. 30% off your normal strike damage is nowhere near a balanced drawback for that - I don’t think even 75% off your normal damage would be, considering that you don’t need to do initial damage to the enemies to apply the DoT. With that kind of add, you don’t need many ticks to drop almost any enemy, and each tick starts to do more damage than many regular strikes.
Second: I tested things on dummies only, so it isn’t completely reliable, but the Fire Walk blast (it doesn’t seem to be related to Kaboom! as I tested both with that Talent and without) does at times apply a very fast DoT tickrate. The faster rate seems irregular, though, and looks like the game somehow applies a couple of extra ticks (with irregular or different from normal) of DoT in between the regular ones. This also seems to be tied to the range from the blast’s center - on both exact hit on a dummy and by hitting the edge of the normal area of the blast the tick rate seemed normal, but on a band between the extra ticks appeared. This does bear further testing (in field conditions) but very much implies bugged behavior.
Even without real field testing this does indeed seems clearly too strong. It would pretty easily be remedied though by fixing (or adjusting) the Fire Walk DoT effect and limiting Famished Flames. As @Exanimia said before, Lingering Flames by itself wasn’t especially strong, and I doubt some extra damage added to that wouldn’t bust anything either, but the huge addition of Famished Flames breaks things.
I initially thought that Famished would raise the DoT damage to 150% of normal (with the regular damage penalty still) and thought that’d be pretty reasonable - boost to some weapons (and Lingering Flames), but really not worth it on many. That it adds 150% damage is honestly broken, and overshadows pretty much everything else on any of Sienna’s weapons that rely even moderately on DoT - which is half her melee selection and almost all staves.
I repeat and emphasize: according to my tests and thoughts, despite what the topic says, Lingering Flames isn’t to blame here. The full DoT build is very strong, but the culprit is primarily Famished Flames, and secondarily the apparently bugged behavior of Fire Walk.