Vel'Koz and Braum are both pretty solidly designed champions with impressive depth, to be sure, but Gnar? Azir? Hell, even Aatrox? Are those champions particularly elegant, as the video put it? Not terribly.
And again, the bigger problems aren't the champions, but the items and map that every player interacts with constantly. They're adding piles upon piles of situational buffs and minor objectives and timings and completely nonsensical components. I mean, hell, I initially supported the support item rework, but I'm looking back on it and it's a change that brought massive complexity but offered almost no choice or depth. That's...not great. Not great at all.
Gnar is annoying and brokenly overpowered, but he is one great design. Fun to play as and against, clear strengths and weaknesses, and a kit that is fairly basic and self explanatory before you get into the nitty gritty stuff. Azir is still somewhat clunky (its obvious that they kinda had to rush him for the Shurima event) but he is insanely fun to play as, and he has one of the most unique and rewarding kits in the game. I get that not every champion has to be like these two, and I think they are pushing it with three hard-to-master champion releases in a row with Kalista, but these complex designs are fun every once in a while.
I will admit Aatrox is cancer. Easily one of Riot's worst designs, both as a standalone kit and a kit for that specific character.
Now, you are exaggerating the significance and difficulty of the preseason objective changes. The only major changes are A) theres a new vision mob in the river, B) some towers have bonus effects, and C) dragon gives different stuff. All of these things aren't huge essential changes to the game, and they don't make the learning curve any harder. Like in the video, this is not stuff that a player needs to know the first time they play, or even in the first 20 times they play. The idea of towers and dragon and baron are all the same, they just have minor tweaks.