Let's see what we have here..
So things started from numerous light mains while barely any dark. That is true. Unfortunately, dark seems to be in a bad period regarding their unit upgrades for a long time at DMM, be it a new hime or a previous getting AW or rebalance. Lots of strong self buffers & whatnots, but lacking something that integrate those into a team. They're still the slowest at DMM with their burst cycles.

Light has been hyped & overhyped for about as long as dark had its.. dark period. No wonders on that, they really suddenly rounded up to be an amazing element from basically nothing in a small time span of half year. Starting from Tish + Mike AW until Iris including old units rebalances as well as great new units. The thing one shouldn't forget that it's been almost a year since Iris' release and light didn't get any relevant new unit meanwhile. Sure Michiru is fun to play with due to ridiculous amount of assault bonuses, but that's about all she brings, VBaal sounds like an amazing unit.. until you realize, light has units that already do every of her ability better, talk about wasted potential. She would have been great for any other element. Oh, and Hastur who was never to be seen after her release. All of them limited & no actual improvements to the element.
However, there was still a rebalance for Athena which made her do her tanking job properly. That, and Eros AW who admittedly turned out great.. against debuffing content. That two has been the only improvement since, and that's almost half a year as well.

A year without new units, and half a year without whatsoever improvement. While the result that it's still a top pick as an element despite that talks for itself, it left plenty of opening for other elements to catch up.

Thunder rounded up to be another great overall pick with most himes being useful in a way or another. It already had TAphro+Marduk AW combo at the time light ascended. That combo could bring normal attacks to a ridiculous level due to the combination of vigour+castle with its pursuit above the already high normal attacks and TAphro's barrier helps keeping it up. Not to mention it's also a way to mitigate incoming damage in order to keep vigour going. Then they got an amazing healer - Dian - who does literally everything a healer needs to and in pretty short intervals due to CD refresh on burst. Jupiter got rebalanced to stupid levels from where she started becoming a pretty reliable unit having speed, nuke & party buff.. oh, and echo. Raiko AW became the damage cutter. Above her CD6 DUR2 cut (30% off-element & 50% vs. water) which gets -2CD every burst, she has party-wide zeal to speed up your units and possibly activating her new passive which further decreases the CDs. And a potent 25% atk down. Nuwa can further increase the already ridiculous normal attacks with crit/cap up + rampage, also having atk/def debuff and giving DATA buff to the whole party with her burst. ... I'm getting tired of inputting wiki info here, point being I'm trying to look for a thunder hime without much of a use, but I really have to look, then I find uhh.. Halloween Mike, Mammon utterly useless & Tyr AW, Ymir hard to find a use.

Then we got wind which also been getting more & more potent since and after Azzy AW (for the record, let's go through this most borken unit beside Mike AW again: CD7 DUR5 50% atk/def buff 50% DATA and a meager 5% DoT, and another CD7 DUR5 30% cut, 300 drain, 500 regen, both of them available party-wide). Wind became rather nuke heavy the past months which made them the rulers of the recent short Dummies and probably going to do great at tower as well while their DPS & overall utilities also improved a lot.

Water showing signs of improvement as well still having amazing defensive measures and Diana doing similar job as TAphro, Pluto providing stackable DATA buff (meaning that it also stacks with normal DATA buffs), Shiva AW doing nukes & party damage buffing and Cthulhu AW doing an amazing job at cucking the boss from doing damage and party BG while at it.

Finally, fire.. not changing much, still doing great in the kill before getting killed space whale realm.