Good champs that can use it(in my opinion).
Thunderlords works if you need the extra burst or the early game poke, but buffing your soldiers with a teemo e, applying tick damage late game with rapid auto attacks just adds more damage and possibly finishing people off is also fun(it is also for late game like Azir).
Setting people on fire twice and reapplying it every tick? Why not? Thunderlord's damage is outclassed by his own passive anyway and this he doesn't need to burst someone with his entire kit in order to use it, unlike thunderlords.
Turret attacks can apply it rapidly, effectively giving them extra damage per attack. Between this and Thunderlords it is merely a matter of preference.
another champ who can proc thunderlords, but it requires most of her kit to do so. Dft is an ok alternative for a damage Karma.
Having to drop 200+ mana in order to use thunderlords is rather dumb, Dft is better for her as it allows her to poke.
Dot mages love it, as their dot's usually last longer than the single target spell duration of Dft does.
Rarely you will be able to proc thunderlords, as she needs to q and aa twice(does her powerchord count as a separate hit?) in order to proc it, or q, r, aa, and if you need to put yourself in unnecessary danger or use a long cooldown ult in order to proc thunderlords, you shouldn't take it.
The only way you are going to proc thunderlords with Vigar is if you aa(which is dumb with him as you are both squishy and a prime target) or you use q,w, and his r. Seeing as his r is a execute the person is probably dead already if you are using it. And after ~250 ap Dft out dps's thunderlords, and after close to 700 ap(or higher) it can out damage it with one application. And 700 ap to a good Veigar is nothing.
Another long range poke mage, another champ that would need to drop their entire kit in order to use Thunderlords, and with Xerath, you do not want to be in aa range if possible.
Ziggs can use both without a clear preference for either. His e can proc Thunderlords by itself, but his q is on a short cooldown for frequent applications of Dft. So it would be up to you.
60% of ad on Dft is nothing to sneeze at(30% on his q and w) and since Jhin can get 400-700 ad easily it is rather good on him. Burning people for 248-428 over 4 seconds when you use your w or you ult is wonderful. And a 124-214 boost to your traps and grenade is decent.
I have seen (and died because of) Dft on a Mf, and as a caster adc it makes sense on her. This one is up to preference though.
So yah, anyone without an easy application of thunderlords, cannot use the stormriders surge well, or is not tank/aa focused(and not support either) usually goes Dft.