Reno Jackson is godlike. I will not stand for this slander.
GvG and TGT class Legendaries were pretty mediocre overall. Old Gods class Legendaries were an improvement.
Xaril, Fandral, Malkorok, Hallazeal, Lightlord are all usable to staple cards. I personally think Huhuran is a good card too and has potential to see more play in the future. Chogall may have potential too in the future with expensive Warlock spells printed. Anomalus is a terrible card and Herald may have a future if they completely redesign Priest to be more aggressive and board centric but aside from that it also sucks.
Compare it to TGT:
Acidmaw/Dreadscale : I think Dreadscale is awesome but should've been a neutral. Acidmaw sucks, it's like a super expensive Equality clear peace.
Mistcaller: Sucks
Rhonin: Too slow even though it's not actually awful.
Varian: Kinda usable in Tempo based Warrior decks.
Anuburak: Usable in Control Rogue if that was a thing, otherwise unplayable.
Wilfred: Sucks.
Confessor Paeltress: Slow and RNG based, Ysera is usually a lot better.
Eadric: Alright card but not too great. You would just rather clear the board than neuter it.
Aviana: Too slow.
GvG was even worse, only Voljin and Malganis were usable. Gallywix is kinda usable in the Thief Rogue though. Leviathan, Gazrilla, Bolvar, Iron Juggernaut and Malorne all sucked.
So yeah Blizzard has to try a lot harder on the class Legendaries. They still don't even come close to touching the classic class Legendaries.
Looking back at it, Old Gods was easily the best expansion that Blizzard put out of the 3. GvG was the start of the RNG clown fiesta with absurd power creep cards and like half of the cards were mech based (so if your class couldn't play a Mech deck GvG sucked for you). TGT was the opposite, they introduced a cool mechanic but just about everything in TGT was very weak aside from a couple of cards like Tuskar and Mysterious Challenger that somehow did not get the same treatment. It was a good expansion for Shamans but a mediocre one for everyone else. Old Gods felt more balanced, less RNG and power creep than GVG but not as irrelevant as TGT plus a majority of the class Legendaries were at least good.